virtio: document ENOSPC

drivers handle ENOSPC specially since it's an error one can
get from a working VQ. Document the semantics.

Message-Id: <[e-mail]>
Reported-by: Parav Pandit <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Reviewed-by: Stefan Hajnoczi <[e-mail]>
Reviewed-by: Parav Pandit <[e-mail]>

drivers/virtio/virtio_ring.c

···
  * at the same time (except where noted).
  *
  * Returns zero or a negative error (ie. ENOSPC, ENOMEM, EIO).
+ *
+ * NB: ENOSPC is a special code that is only returned on an attempt to add a
+ * buffer to a full VQ. It indicates that some buffers are outstanding and that
+ * the operation can be retried after some buffers have been used.
  */
 int virtqueue_add_sgs(struct virtqueue *_vq,
 		      struct scatterlist *sgs[],