server: document wl_event_loop and wl_event_source

This documents all the public API related to wl_event_loop and
wl_event_source objects.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
[Pekka: fixed typos pointed by Yong]
[Pekka: fixed typos pointed by Christopher]
Reviewed-By: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>