docs: kdoc: split struct-member rewriting out of dump_struct()

docs: kdoc: split struct-member rewriting out of dump_struct()

The massive loop that massages struct members shares no data with the rest
of dump_struct(); split it out into its own function.  Code movement only,
no other changes.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250807211639.47286-7-corbet@lwn.net