docs: kdoc: drop "sectionlist"

docs: kdoc: drop "sectionlist"

Python dicts (as of 3.7) are guaranteed to remember the insertion order of
items, so we do not need a separate list for that purpose.  Drop the
per-entry sectionlist variable and just rely on native dict ordering.

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