DOC: Suppress warnings in visualization.rst

Closes #8234. Sorry to take a long.

I understand there are 2 warnings:

1. Warning in /Users/sin/Documents/Git/pandas/doc/source/visualization.rst at block ending on line 1275
   This is raised from `mpl` as plotting empty axes for demonstration purpose. Thus simply suppressed.

2. Warning in /Users/sin/Documents/Git/pandas/doc/source/visualization.rst at block ending on line 1312
   This is a warning displayed when passing multiple axes. Added explanations.
maybe better update the text above (line 1300) instead of putting a comment, as that "the keywords are ignored" is not really correct: you have to put them at False.

I am wondering, isn't there a way to do this automatically (so it is not needed that the user has to specify `sharex=False`), but maybe that is not worth the extra complexity.