compositor: document weston_view_damage_below()

Explains what weston_view::clip is.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
diff --git a/src/compositor.h b/src/compositor.h
index 5c0ea74..47b6036 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -761,7 +761,7 @@
 	struct weston_plane *plane;
 	struct weston_view *parent_view;
 
-	pixman_region32_t clip;
+	pixman_region32_t clip;          /* See weston_view_damage_below() */
 	float alpha;                     /* part of geometry, see below */
 
 	void *renderer_state;