docs: kdoc: backslashectomy in kdoc_parser

docs: kdoc: backslashectomy in kdoc_parser

A lot of the regular expressions in this file have extraneous backslashes
that may have been needed in Perl, but aren't helpful here.  Take them out
to reduce slightly the visual noise.

Escaping of (){}[] has been left in place, even when unnecessary, for
visual clarity.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250807211639.47286-4-corbet@lwn.net