DOC: some hacks to get rid of warnings
This PR includes some hacks to numpydoc to get rid of a bunch of warnings (split of from https://github.com/pydata/pandas/pull/11069 as I want these merged for 0.17.0). There is some more explanation in the commit messages.

Closes #6100

The disadvantage of this is that this causes a deviation from numpydoc upstream, and makes it more difficult to update to that one.
