docs: kdoc: move the prefix transforms out of dump_struct()

docs: kdoc: move the prefix transforms out of dump_struct()

dump_struct is one of the longest functions in the kdoc_parser class,
making it hard to read and reason about.  Move the definition of the prefix
transformations out of the function, join them with the definition of
"attribute" (which was defined at the top of the file but only used here),
and reformat the code slightly for shorter line widths.

Just code movement in the end.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250807211639.47286-5-corbet@lwn.net