Document module privacy
We should better document what in pandas is considered private.

http://pandas.pydata.org/pandas-docs/stable/whatsnew.html#reorganization-of-the-library-privacy-changes should be captured somewhere more permanent than in a whatsnew release. source is at https://github.com/pandas-dev/pandas/blob/master/doc/source/whatsnew/v0.20.0.txt#L1194

I think a new section in http://pandas-docs.github.io/pandas-docs-travis/overview.html is the best place for it.