docstrings/sphinx: Support "Warns" sections

[pawamoy]

Is your feature request related to a problem? Please describe.
The goal is to have a common set of features between all parsers.

Describe the solution you'd like
The Sphinx parser should be able to parse "Warns" sections, if it makes any sense.

[pawamoy]

Closing for cleanup, but if you'd like to work on this, just ask and I'll reopen!