DOC/CLN: Cleanups plotting.py
- Some refactoring which should not affects to output.
- Made `DataFrame.plot` and `Series.plot` should have consistent docstring except differences.

@sinhrks Docstrings are looking nice! Made some comments (and note that a lot of them are comments on the existing docstrings, but thought was a good opportunity to look at that too)

I did not yet really look at the actual code refactor, but the only think I was worrying a bit is: you changed the order of keyword arguments, that is right? Now you put `kind` second (which is very logical), before `x` and `y`. But I was wondering how many people would rely on the position of `x` and `y`? (for the others that doesn't matter I think, but for those first keyword, it does maybe)
 Thanks, fixed the points. And agreed to hold `x` and `y` in the current arg order.
 @sinhrks Can you rebase this? @TomAugspurger Can you also have a look?
 Yep, rebased.
 This cleanup really needed to be done, thanks @sinhrks. Just had a few inline comments. There's also a few lines in the docstring that are longer that 80 characters, but if this is a formatting thing so that it looks nice in the REPL, then no problem.
 @sinhrks pls rebase
 @sinhrks I can merge once you get a chance to rebase. Let me know.
 @sinhrks status?
 @sinhrks will you have a chance to do this in the next couple days? If not I can cherry pick things. I'd like to get all the vis PRs done by tomorrow.
 @TomAugspurger let's either cherry-pick or push to 0.15.1 whatever you can do in next couple of days
