docs: versionadded for kwargs argument

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
I think we should use `versionchanged` here (as we did in `dictdiffer`), which seems better indicated for additional arguments.  The philosophy is:

- versionadded: new feature added, e.g. new API function
- versionchanged: existing feature amended, e.g. new parameters, e.g. new side effects

See http://sphinx-doc.org/markup/para.html

I'll amend this while merging...