compositor: fix comments about weston_compositor::surface_list

compositor: fix comments about weston_compositor::surface_list

a7af70436b7dccfacd736626d6719b3e751fd985 converted the surface list into
a view list. There is no weston_compositor::surface_list anymore.

It looks like weston_surface::output's comment about surface list does
not apply to view list. Still, many places assume weston_surface::output
is not NULL when processing "visible" surfaces, e.g. those reachable via
the view list.

The comment on weston_view::output is updated. It seems there is no
longer any requirement for it to be NULL if the view is not in
view_list.

weston_view::link is documented to be in weston_compositor::view_list,
and weston_compositor::view_list is documented to contain weston_views.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>
[Pekka: removed also the other "Must be NULL" comment.]