vhost-user.rst: specify vhost-user back-end action on GET_VRING_BASE

By default, we assume that server need to wait all inflight IO on
GET_VRING_BASE. However, this fact is not recorded anywhere in the
documentation.
So, add this info in rst.

Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260115081103.655749-2-dtalexundeer@yandex-team.ru>

docs/interop/vhost-user.rst

@@ -1250,7 +1250,8 @@ Front-end message types
 
   When and as long as all of a device's vrings are stopped, it is
   *suspended*, see :ref:`Suspended device state
-  <suspended_device_state>`.
+  <suspended_device_state>`. The back-end must complete all inflight I/O
+  requests for the specified vring before stopping it.
 
   The request payload's *num* field is currently reserved and must be
   set to 0.