scripts/kernel-doc.py: better handle empty sections

scripts/kernel-doc.py: better handle empty sections

While doing the conversion, we opted to skip empty sections
(description, return), but this makes harder to see the differences
between kernel-doc (Perl) and kernel-doc.py.

Also, the logic doesn't always work properly. So, change the
way this is done by adding an extra step to remove such
sections, doing it only for Return and Description.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/1b057092a48ba61d92a411f4f6d505b802913785.1744106241.git.mchehab+huawei@kernel.org