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.
I think this can be a bit clearer. 
Maybe, for the types, make it shorter (something like `DataFrame, Series, array-like or string`. does array-like include lists?) and then explain the different options a bit more on the explanation lines.

Also the "matching labels" is not clear to me that it are the column names (?) that should match?