doc: Cope with Sphinx logging deprecations

Recent versions of sphinx will emit messages like:

  Documentation/sphinx/kerneldoc.py:103:
     RemovedInSphinx20Warning: app.warning() is now deprecated.
     Use sphinx.util.logging instead.

Switch to sphinx.util.logging to make this unsightly message go away.
Alas, that interface was only added in version 1.6, so we have to add a
version check to keep things working with older sphinxes.

Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>