[media] media: Improve documentation for link_setup/link_modify

Those callbacks are called with the media_device.graph_mutex held.

Add a note about that, as the code called by those notifiers should
not be touching in the mutex.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

include/media/media-entity.h

@@ -179,6 +179,9 @@ struct media_pad {
  * @link_validate:	Return whether a link is valid from the entity point of
  *			view. The media_entity_pipeline_start() function
  *			validates all links by calling this operation. Optional.
+ *
+ * Note: Those these callbacks are called with struct media_device.@graph_mutex
+ * mutex held.
  */
 struct media_entity_operations {
 	int (*link_setup)(struct media_entity *entity,