compositor: surface and view output comment fixes

compositor: surface and view output comment fixes

weston_surface::output and weston_view::output as used for different
purposes. Only the surface output is used for frame callbacks.

The uses of the view output are much more vague and hard to describe.

Also fix a comment mistake in weston_surface_assign_output().

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: more verbose on the compositor.h comments]