DOC: Some doc cleanup
Some minor stuff regarding ``.ix`` and ``.loc``.

1:
The [README](https://github.com/pandas-dev/pandas) has a link about ``DataFrame.ix``. As ``ix`` is deprecated, the link is changed to [selection-by-label](https://pandas.pydata.org/pandas-docs/stable/indexing.html#selection-by-label).

2:
In [selection-by-label](https://pandas.pydata.org/pandas-docs/stable/indexing.html#selection-by-label) a minor wall of warnings come before the reader even knows what the warning are about (because this is the section about `´loc.``...). This is very "scary" introduction.

I've moved the chained-assignment warning to after the intro, and the rest to the end of the section, so the reader first gets an understanding about ``loc`` in general and then only after that gets the warnings about the pitfalls of ``loc``.
I would have preferred that you broke your in PR into two separate PR's: one for the indexing and one for your added tests.  It's a little easier to review, and helps to main the atomicity of the PR's / merges. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19968?src=pr&el=h1) Report
> Merging [#19968](https://codecov.io/gh/pandas-dev/pandas/pull/19968?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fd010de51bd21e4240daac2e1fd0044c65a45091?src=pr&el=desc) will **decrease** coverage by `0.06%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19968/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19968?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19968      +/-   ##
==========================================
- Coverage   91.71%   91.65%   -0.07%     
==========================================
  Files         150      150              
  Lines       49104    49049      -55     
==========================================
- Hits        45035    44954      -81     
- Misses       4069     4095      +26
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.03% <ø> (-0.07%)` | :arrow_down: |
| #single | `41.79% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19968?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbWlzc2luZy5weQ==) | `85.9% <0%> (-5.75%)` | :arrow_down: |
| [pandas/plotting/\_timeseries.py](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL190aW1lc2VyaWVzLnB5) | `60.82% <0%> (-4.49%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.59%)` | :arrow_down: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `92.44% <0%> (-0.65%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.59% <0%> (-0.07%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.66% <0%> (ø)` | :arrow_up: |
| [pandas/compat/pickle\_compat.py](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9waWNrbGVfY29tcGF0LnB5) | `75.6% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.02% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.23% <0%> (ø)` | :arrow_up: |
| ... and [7 more](https://codecov.io/gh/pandas-dev/pandas/pull/19968/diff?src=pr&el=tree-more) | |

------

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