libvhost-user: improve vu_queue_pop() doc

Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>

contrib/libvhost-user/libvhost-user.h

@@ -358,7 +358,8 @@ void vu_queue_notify(VuDev *dev, VuVirtq *vq);
  * @vq: a VuVirtq queue
  * @sz: the size of struct to return (must be >= VuVirtqElement)
  *
- * Returns: a VuVirtqElement filled from the queue or NULL.
+ * Returns: a VuVirtqElement filled from the queue or NULL. The
+ * returned element must be free()-d by the caller.
  */
 void *vu_queue_pop(VuDev *dev, VuVirtq *vq, size_t sz);