docs: kdoc: clean up check_sections()

docs: kdoc: clean up check_sections()

entry.sectcheck is just a duplicate of our list of sections that is only
passed to check_sections(); its main purpose seems to be to avoid checking
the special named sections.  Rework check_sections() to not use that field
(which is then deleted), tocheck for the known sections directly, and
tighten up the logic in general.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>