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.
This is list getting rather long ..
What do you think of just doing `kind : string, default 'line'` ? As all possibilities are also listed underneath? (and maybe in that case put them in quotes in the list like `- 'bar' : vertical bar plot`, then it is also clear the those strings are the options)