util: Document wl_list methods

Add doxygen comment blocks to all wl_list methods.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
[Pekka: removed "itself" as suggested by Bryce]
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>