doc/sphinx: Enable keep_warnings

Unfortunately warnings generated after parsing in sphinx can end up
with entirely bogus files and line numbers as sources. Strangely for
outright errors this is not a problem. Trying to convert warnings to
errors also doesn't fix it.

The only way to get useful output out of sphinx to be able to root
cause the error seems to be enabling keep_warnings, which inserts
a System Message into the actual output. Not pretty at all, but I
don't really want to fix up core rst/sphinx code, and this gets the job
done meanwhile.

Cc: Markus Heiser <markus.heiser@darmarit.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>