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.

squash / merge when ready
 I would like to not squash the HACKs, as it are separate changes with separate explanations in the commit messages, and so it is more easy to see what we exactly changes and more easy to revert them separately if things change in numpydoc.
 sure no prob 
