[media] v4l: vsp1: Document calling context of vsp1_pipeline_propagate_alpha()

The function can only be called from a s_stream handler as it requires a
valid display list context (due to calling vsp1_uds_set_alpha() which
writes to module registers). Document the requirement.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/platform/vsp1/vsp1_pipe.c

@@ -318,6 +318,9 @@ void vsp1_pipeline_frame_end(struct vsp1_pipeline *pipe)
  * to be scaled, we disable alpha scaling when the UDS input has a fixed alpha
  * value. The UDS then outputs a fixed alpha value which needs to be programmed
  * from the input RPF alpha.
+ *
+ * This function can only be called from a subdev s_stream handler as it
+ * requires a valid display list context.
  */
 void vsp1_pipeline_propagate_alpha(struct vsp1_pipeline *pipe,
 				   struct vsp1_entity *input,