docs: kernel-doc: Finish moving STATE_* code out of process_file()

Move STATE_INLINE and STATE_DOCBLOCK code out of process_file(), which now
actually fits on a single screen.  Delete an unused variable and add a
couple of comments while I'm at it.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>