docproc: add support for reStructuredText format via --rst option

Expect reStructuredText input and have kernel-doc produce
reStructuredText output with the new --rst option. Also add --docbook
option for completeness. If no option is given, default to
reStructuredText if the input file has ".rst" extension, DocBook
otherwise.

Directives for reStructuredText use .. ! instead of just !, to make them
reStructuredText comments.

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