compositor: add a doc note for weston_view::parent_view

It is used by sub-surfaces only, for fetching the root view's
weston_layer.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-By: Giulio Camuffo <giuliocamuffo@gmail.com>
1 file changed