[media] videobuf2-v4l2: document two helper functions

Document vb2_ops_wait_prepare() and vb2_ops_wait_finish(),
in order to fix those two warnings:
Documentation/media/kapi/v4l2-dev.rst:166: WARNING: c:func reference target not found: vb2_ops_wait_prepare
Documentation/media/kapi/v4l2-dev.rst:166: WARNING: c:func reference target not found: vb2_ops_wait_finish

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

include/media/videobuf2-v4l2.h

···
 		unsigned long len, unsigned long pgoff, unsigned long flags);
 #endif
 
-/* struct vb2_ops helpers, only use if vq->lock is non-NULL. */
-
+/**
+ * vb2_ops_wait_prepare - helper function to lock a struct &vb2_queue
+ *
+ * @vq: pointer to struct vb2_queue
+ *
+ * ..note:: only use if vq->lock is non-NULL.
+ */
 void vb2_ops_wait_prepare(struct vb2_queue *vq);
+
+/**
+ * vb2_ops_wait_finish - helper function to unlock a struct &vb2_queue
+ *
+ * @vq: pointer to struct vb2_queue
+ *
+ * ..note:: only use if vq->lock is non-NULL.
+ */
 void vb2_ops_wait_finish(struct vb2_queue *vq);
 
 #endif /* _MEDIA_VIDEOBUF2_V4L2_H */