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``.