docs: kdoc: separate out the handling of the declaration phase

docs: kdoc: separate out the handling of the declaration phase

The BODY_MAYBE state really describes the "we are in a declaration" state.
Rename it accordingly, and split the handling of this state out from that
of the other BODY* states.  This change introduces a fair amount of
duplicated code that will be coalesced in a later patch.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250621203512.223189-4-corbet@lwn.net