DOC: update the Index.get_level_values 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.get_level_values)  ##################
################################################################################

Return an Index of values for requested level.

Return an Index of values for requested level, equal to the length
of the index.

Parameters
----------
level : int or str
    It is either the integer position of the level in the
    MultiIndex, or the `name` of the level.
    If given as an integer, it must be between 0 and the
    number of levels.

Returns
-------
values : Index
    `self`, as there is only one level in the Index.

See also
---------
pandas.MultiIndex.get_level_values : get values for a level of a
                                     MultiIndex

Notes
---------
For `Index`, level should be 0, since there are no multiple levels.

Examples
---------

Create an Index:

>>> idx = pd.Index(list('abc'))

Get level value by supplying level as integer:

>>> idx.get_level_values(0)
Index(['a', 'b', 'c'], dtype='object')

Create a MultiIndex:

>>> mi = pd.MultiIndex.from_arrays((list('abc'), list('def')))
>>> mi.names = ['level_1', 'level_2']

Get level values by supplying level as name:

>>> mi.get_level_values('level_2')
Index(['d', 'e', 'f'], dtype='object', name='level_2')

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

Docstring for "pandas.Index.get_level_values" correct. :)
```
Reading through this, it may make sense to say that this isn't very useful for non-MultiIndex indexes. A small example should be OK, but the most important bit is the See Also to MultiIndex.get_level_values. Most of the notes have been addressed and force pushed. @bkil-syslogng Do you have time to update the PR based on the last comments? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20210?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@20ca108`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20210/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20210?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20210   +/-   ##
=========================================
  Coverage          ?   91.72%           
=========================================
  Files             ?      150           
  Lines             ?    49162           
  Branches          ?        0           
=========================================
  Hits              ?    45096           
  Misses            ?     4066           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.11% <100%> (?)` | |
| #single | `41.86% <56.25%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20210?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20210/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.66% <100%> (ø)` | |

------

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> **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/20210?src=pr&el=footer). Last update [20ca108...6aa8b77](https://codecov.io/gh/pandas-dev/pandas/pull/20210?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Rebased and added final fixes.

@bkil-syslogng Thanks!