[#943] Add custom docstrings for some 3rd-party stuff

The plugins toolkit imports some things from 3rd-party libraries, e.g.
the request, response and c objects from Pylons. We want to use our own
docstrings for these rather than having the 3rd-party docstrings appear
in the CKAN docs. Also in some cases the 3rd-psrty function has no
docstring, or introspection would end up with a class docstring where
what we actually want is to document a specific object-instance.

So add custom docstring overrides as a feature implemented in toolkit.py
and toolkit_sphinx_extension.py.