media: v4l2-dev.h: add kernel-doc to two macros

There are two macros at v4l2-dev.h that aren't documented.

Document them, for completeness.
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

include/media/v4l2-dev.h

@@ -261,9 +261,21 @@ struct video_device
 	struct mutex *lock;
 };
 
-#define media_entity_to_video_device(__e) \
-	container_of(__e, struct video_device, entity)
-/* dev to video-device */
+/**
+ * media_entity_to_video_device - Returns a &struct video_device from
+ *	the &struct media_entity embedded on it.
+ *
+ * @entity: pointer to &struct media_entity
+ */
+#define media_entity_to_video_device(entity) \
+	container_of(entity, struct video_device, entity)
+
+/**
+ * to_video_device - Returns a &struct video_device from the
+ *	&struct device embedded on it.
+ *
+ * @cd: pointer to &struct device
+ */
 #define to_video_device(cd) container_of(cd, struct video_device, dev)
 
 /**