vringh: explain more about cleaning riov and wiov

riov and wiov can be reused with subsequent calls of vringh_getdesc_*().

Let's add a paragraph in the documentation of these functions to better
explain when riov and wiov need to be cleaned up.

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20210315163450.254396-5-sgarzare@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>