[media] media: entity: Document the media_entity_ops structure

Currently, there's no documentation for the structure. Add a
kernel-doc nano documentation to it.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

include/media/media-entity.h

@@ -44,6 +44,15 @@ struct media_pad {
 	unsigned long flags;		/* Pad flags (MEDIA_PAD_FL_*) */
 };
 
+/**
+ * struct media_entity_operations - Media entity operations
+ * @link_setup:		Notify the entity of link changes. The operation can
+ *			return an error, in which case link setup will be
+ *			cancelled. Optional.
+ * @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.
+ */
 struct media_entity_operations {
 	int (*link_setup)(struct media_entity *entity,
 			  const struct media_pad *local,