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.

@jorisvandenbossche @TomAugspurger ?
 @sinhrks these warnings also appear when running the tests, so probably there are also some of these calls that can be fixed (see https://travis-ci.org/pydata/pandas/jobs/42314766)
 @sinhrks @jorisvandenbossche this ready to merge?
 just the tests, it would be good to take a look at that as well
 @sinhrks can you see if you can fix the test warnings as well?
 @jorisvandenbossche @sinhrks ?
 @jorisvandenbossche ?
 bumbed to 0.16, as you don't visually see it in the docs (warning is not shown by sphinx), so no blocker
 Sure, will fix tests also.
 Updated, and prepared a separate fix #9278 for a program change.
 @jorisvandenbossche OK, now all the warnings should disapear. Pls check.
 Just looking at the travis log, I still see some warnings. Eg (in the log of the third job: https://travis-ci.org/pydata/pandas/jobs/48128130):

```
..................S.................................................../home/travis/build/pydata/pandas/pandas/tools/plotting.py:3225: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared

"is being cleared", UserWarning)

........../home/travis/build/pydata/pandas/pandas/tools/plotting.py:3208: UserWarning: When passing multiple axes, sharex and sharey are ignored.These settings must be specified when creating axes

"These settings must be specified when creating axes", UserWarning)

./home/travis/miniconda/envs/pandas/lib/python2.7/site-packages/matplotlib/axes.py:4747: UserWarning: No labeled objects found. Use label='...' kwarg on individual plots.

warnings.warn("No labeled objects found. "
```

It is not clear to me if you can see which test triggered the warnings.
 @sinhrks add some questions

@TomAugspurger can you also give this a brief look?
 @TomAugspurger Thanks to confirm. I think everything done now.
 OK. I've only been able to give it a quick glance, but from that everything looks fine.
 Regarding my comment above (https://github.com/pydata/pandas/pull/8877#issuecomment-71401640) the `No labeled objects found. Use label='...' kwarg on individual plots.` is still present in the tests, but this is more a leftover not addressed in #9278, so see if you can look at it
 Yes, will do. I originally misunderstood you're referring to different line.
 @sinhrks This is ready to merge for you?

In any case, thanks for your endurance, this has been a long one ..
 @jorisvandenbossche Yes. I expect all warnings should be disappears after this and #9464, otherwise will implement a fix in #9464.
 Thanks! 
Maybe then rebase #9464 to be sure 
