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.
OK. One option is to always disable the warning. But it might be unexpected behaviour if someone pass `sharex=True` explicitly.