util: Improve documentation of wl_argument

Standardize the doxygen comment format, add clarity to the writing, decouple
the description from specifics of usage, add see-also's, and massage the union
member type comments.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>