docs: kdoc: consolidate the stripping of private struct/union members

docs: kdoc: consolidate the stripping of private struct/union members

There were two locations duplicating the logic of stripping private members
and associated comments; coalesce them into one, and add some comments
describing what's going on.

Output change: we now no longer add extraneous white space around macro
definitions.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250807211639.47286-2-corbet@lwn.net