docs: kdoc_parser: add data/function attributes to ignore

docs: kdoc_parser: add data/function attributes to ignore

Recognize and ignore __rcu (in struct members), __private (in struct
members), and __always_unused (in function parameters) to prevent
kernel-doc warnings:

  Warning: include/linux/rethook.h:38 struct member 'void (__rcu *handler' not described in 'rethook'
  Warning: include/linux/hrtimer_types.h:47 Invalid param: enum hrtimer_restart (*__private function)(struct hrtimer *)
  Warning: security/ipe/hooks.c:81 function parameter '__always_unused' not described in 'ipe_mmap_file'
  Warning: security/ipe/hooks.c:109 function parameter '__always_unused' not described in 'ipe_file_mprotect'

There are more of these (in compiler_types.h, compiler_attributes.h)
that can be added as needed.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251127063117.150384-1-rdunlap@infradead.org>