DOC: update the Index.isin docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [x] PR title is "DOC: update the <your-function-or-method> docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [x] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [x] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```

################################################################################
######################## Docstring (pandas.Index.isin)  ########################
################################################################################

Boolean array on existence of index values in `values`.

Compute boolean array of whether each index value is found in the
passed set of values. Length of the returned boolean array matches
the length of the index.

Parameters
----------
values : set or list-like
    Sought values.

    .. versionadded:: 0.18.1

    Support for values as a set.

level : str or int, optional in the case of Index, compulsory on
    MultiIndex
    Name or position of the index level to use (if the index is a
    MultiIndex).

Returns
-------
is_contained : ndarray (boolean dtype)

See also
--------
DatetimeIndex.isin : an Index of :class:`Datetime` s
TimedeltaIndex : an Index of :class:`Timedelta` s
PeriodIndex : an Index of :class:`Period` s
MultiIndex.isin : Same for `MultiIndex`
NumericIndex.isin : Same for `Int64Index`, `UInt64Index`,
                    `Float64Index`

Notes
-----
If `level` is specified:

- if it is the name of one *and only one* index level, use that level;
- otherwise it should be a number indicating level position.

Examples
--------
>>> idx = pd.Index([1,2,3])
>>> idx
Int64Index([1, 2, 3], dtype='int64')

Check whether a value is in the Index:
>>> idx.isin([1])
array([ True, False, False])

>>> midx = pd.MultiIndex.from_arrays([[1,2,3],
...                                    ['red','blue','green']],
...                                    names=('number', 'color'))
>>> midx
MultiIndex(levels=[[1, 2, 3], ['blue', 'green', 'red']], labels=[[0, 1, 2], [2, 0, 1]], names=['number', 'color'])

Check whether a string index value is in the 'color' level of the
MultiIndex:

>>> midx.isin(['red'],'color')
array([ True, False, False])

>>> dates = ['3/11/2000', '3/12/2000', '3/13/2000']
>>> dti = pd.to_datetime(dates)
>>> dti
DatetimeIndex(['2000-03-11', '2000-03-12', '2000-03-13'], dtype='datetime64[ns]', freq=None)

Check whether a datetime index value is in the DatetimeIndex:

>>> dti.isin(['3/11/2000'])
array([ True, False, False])

################################################################################
################################## Validation ##################################
################################################################################

Docstring for "pandas.Index.isin" correct. :)
```

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.

@noemielteto Do you have time to update the PR based on the feedback? @jorisvandenbossche Yes, I am going to update it today. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20249?src=pr&el=h1) Report
> Merging [#20249](https://codecov.io/gh/pandas-dev/pandas/pull/20249?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/670c2e437e90c08105b2d83df868fc8c6c668a3b?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20249/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20249?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20249      +/-   ##
==========================================
- Coverage   91.79%   91.77%   -0.03%     
==========================================
  Files         152      152              
  Lines       49205    49205              
==========================================
- Hits        45169    45159      -10     
- Misses       4036     4046      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.16% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.85% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20249?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20249/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.68% <ø> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20249/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20249/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.95% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20249?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20249?src=pr&el=footer). Last update [670c2e4...25d97ec](https://codecov.io/gh/pandas-dev/pandas/pull/20249?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Rephrased the examples slightly. Instead of saying "check whether a string is in the index", we say "check whether each value in the index is in a list of strings." Subtle difference, but "string in index" is more like `foo in Index` or `Index.contains(foo)`.

Thanks @noemielteto! @TomAugspurger Totally agree, thanks! @TomAugspurger Just wondering - what is this 15mb file that got merged in with the pull request?

https://github.com/pandas-dev/pandas/blob/master/pd

 @emesterhazy : That's a file that no longer exists 😄 (it should never have been there, not sure why it wasn't caught before merging). Hmm.

@gfyoung did 7d5e653 remove it from the git history?

 I believe it's still present.

cc @jreback @jorisvandenbossche.

We have 

```
* 81e0f6908 - (HEAD -> master, origin/master, origin/HEAD) Bug: Allow np.timedelta64 objects to index TimedeltaIndex (#20408) (53 minutes ago) <Matthew Roeschke>
* 0722af8b1 - DOC: add disallowing of Series construction of len-1 list with index to whatsnew (#20392) (59 minutes ago) <Joris Van den Bossche>
* 7d5e65343 - MAINT: Remove weird pd file (6 hours ago) <gfyoung>
* 4a43815d9 - DOC: update the Index.isin docstring (#20249) (13 hours ago) <Noémi Éltető>
```

Merge end (newest on top)

The size of a fresh clone:

```python
$ git count-objects -vH
count: 0
size: 0 bytes
in-pack: 145618
packs: 1
size-pack: 120.35 MiB
prune-packable: 0
garbage: 0
size-garbage: 0 bytes
```

Cleanup with bfg

```
$ bfg --delete-files pd

Using repo : /private/tmp/pandas/.git

Found 1212 objects to protect
Found 85 tag-pointing refs : refs/tags/debian/0.4.0-1, refs/tags/debian/0.4.1-1, refs/tags/debian/0.4.3-1, ...
Found 16 commit-pointing refs : HEAD, refs/heads/master, refs/remotes/origin/0.19.x, ...

Protected commits
-----------------

These are your protected commits, and so their contents will NOT be altered:

 * commit 81e0f690 (protected by 'HEAD')

Cleaning
--------

Found 17369 commits
Cleaning commits:       100% (17369/17369)
Cleaning commits completed in 1,628 ms.

Updating 2 Refs
---------------

	Ref                          Before     After
	------------------------------------------------
	refs/heads/master          | 81e0f690 | f4343b31
	refs/remotes/origin/master | 81e0f690 | f4343b31

Updating references:    100% (2/2)
...Ref update completed in 18 ms.

Commit Tree-Dirt History
------------------------

	Earliest                                              Latest
	|                                                          |
	...........................................................D

	D = dirty commits (file tree fixed)
	m = modified commits (commit message or parents changed)
	. = clean commits (no changes to file tree)

	                        Before     After
	-------------------------------------------
	First modified commit | 4a43815d | ea22c025
	Last dirty commit     | 4a43815d | ea22c025

Deleted files
-------------

	Filename   Git id
	-----------------------------
	pd       | 8a8a16ad (15.2 MB)


In total, 5 object ids were changed. Full details are logged here:

	/private/tmp/pandas.bfg-report/2018-03-19/06-03-42

BFG run is complete! When ready, run: git reflog expire --expire=now --all && git gc --prune=now --aggressive


--
You can rewrite history in Git - don't let Trump do it for real!
Trump's administration has lied consistently, to make people give up on ever
being told the truth. Don't give up: https://github.com/bkeepers/stop-trump
--
``` Here's the log after

```
* f4343b312 - (HEAD -> master, origin/master, origin/HEAD) Bug: Allow np.timedelta64 objects to index TimedeltaIndex (#20408) (61 minutes ago) <Matthew Roeschke>
* 9dbb89cc5 - DOC: add disallowing of Series construction of len-1 list with index to whatsnew (#20392) (67 minutes ago) <Joris Van den Bossche>
* 3e14b8d7c - MAINT: Remove weird pd file (6 hours ago) <gfyoung>
* ea22c025d - DOC: update the Index.isin docstring (#20249) (13 hours ago) <Noémi Éltető>
```

Do we want to force push that to master? That'll cause some short term pain probably, but I think it's for the best.

I'll wait for a +1, and I suspect @jreback will have to temporarily disable branch protection on master. @TomAugspurger do you want to do a force push to master? I'm trying to read a bit more about how that messes up downstream. It seems to be a pretty bad idea. FYI, those outputs I posted are out of date due to https://github.com/pandas-dev/pandas/pull/20402 being merged. I re-ran bfg with latest master including that change.

<details>

```
Using repo : /private/tmp/pandas/.git

Found 1212 objects to protect
Found 85 tag-pointing refs : refs/tags/debian/0.4.0-1, refs/tags/debian/0.4.1-1, refs/tags/debian/0.4.3-1, ...
Found 16 commit-pointing refs : HEAD, refs/heads/master, refs/remotes/origin/0.19.x, ...

Protected commits
-----------------

These are your protected commits, and so their contents will NOT be altered:

 * commit ee45e05d (protected by 'HEAD')

Cleaning
--------

Found 17370 commits
Cleaning commits:       100% (17370/17370)
Cleaning commits completed in 2,007 ms.

Updating 2 Refs
---------------

	Ref                          Before     After
	------------------------------------------------
	refs/heads/master          | ee45e05d | 7273ea07
	refs/remotes/origin/master | ee45e05d | 7273ea07

Updating references:    100% (2/2)
...Ref update completed in 20 ms.

Commit Tree-Dirt History
------------------------

	Earliest                                              Latest
	|                                                          |
	...........................................................D

	D = dirty commits (file tree fixed)
	m = modified commits (commit message or parents changed)
	. = clean commits (no changes to file tree)

	                        Before     After
	-------------------------------------------
	First modified commit | 4a43815d | ea22c025
	Last dirty commit     | 4a43815d | ea22c025

Deleted files
-------------

	Filename   Git id
	-----------------------------
	pd       | 8a8a16ad (15.2 MB)


In total, 6 object ids were changed. Full details are logged here:

	/private/tmp/pandas.bfg-report/2018-03-19/07-51-40

BFG run is complete! When ready, run: git reflog expire --expire=now --all && git gc --prune=now --aggressive


--
You can rewrite history in Git - don't let Trump do it for real!
Trump's administration has lied consistently, to make people give up on ever
being told the truth. Don't give up: https://www.theguardian.com/us-news/trump-administration
--
```

</details>

And here's the new log

```
* 7273ea070 - (HEAD -> master, origin/master, origin/HEAD) DOC: Only use ~ in class links to hide prefixes. (#20402) (2 hours ago) <Israel Saeta Pérez>
* f4343b312 - Bug: Allow np.timedelta64 objects to index TimedeltaIndex (#20408) (3 hours ago) <Matthew Roeschke>
* 9dbb89cc5 - DOC: add disallowing of Series construction of len-1 list with index to whatsnew (#20392) (3 hours ago) <Joris Van den Bossche>
* 3e14b8d7c - MAINT: Remove weird pd file (8 hours ago) <gfyoung>
* ea22c025d - DOC: update the Index.isin docstring (#20249) (15 hours ago) <Noémi Éltető>
```

I haven't force pushed yet. @TomAugspurger ok to force push. I did this once before a long time ago. just be careful! you may have to set the no-force push flag (in settings), then do it, then revert the flag Done.

Re-enabled branch protection on master. Could someone check the settings? I think that's how they were before. Everything looks ok @TomAugspurger : Yes, I did remove that file in that commit. Yeah, it was still around in the git history though. So I used [bfg](https://rtyley.github.io/bfg-repo-cleaner/) to purge it from the history (rewriting master) and force pushed.

We should probably have some kind of policy around that. OK to do if you notice it right away and master hasn't changed? Wait for a +1 otherwise? > We should probably have some kind of policy around that. OK to do if you notice it right away and master hasn't changed? Wait for a +1 otherwise?

That makes sense.