docs: kdoc: Centralize handling of the item section list

docs: kdoc: Centralize handling of the item section list

The section list always comes directly from the under-construction entry
and is used uniformly.  Formalize section handling in the KdocItem class,
and have output_declaration() load the sections directly from the entry,
eliminating a lot of duplicated, verbose parameters.

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