ENH/DOC: Cleanup docstrings on NDFrame
Resolves much of #4717. Basically cleans up docstrings in a few places
plus, for the few functions that need them, uses a shared dict of
function definitions to share them. This has the side-benefit of letting
the function docstrings reside right next to the functions themselves,
maintaining readability.

@jreback what do you think? I think this is nicely succinct and easy to follow.
 find by me (just those clipboard comments) above
 so it's just pyperclip for OSX/Windows?
 pyperclip IIRC tries various methods eg xclip,xsel, qt on Linux; native access in windows and says it works on Mac which I think was verified 

it is included in pandas/utils IIRC
