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>

src/compositor.h

@@ -759,6 +759,8 @@ struct weston_view {
 	struct wl_list link;
 	struct weston_layer_entry layer_link;
 	struct weston_plane *plane;
+
+	/* For weston_layer inheritance from another view */
 	struct weston_view *parent_view;
 
 	pixman_region32_t clip;          /* See weston_view_damage_below() */