docs: avoid vhost-user-net specifics in multiqueue section

The "Multiple queue support" section makes references to vhost-user-net
"queue pairs".  This is confusing for two reasons:
1. This actually applies to all device types, not just vhost-user-net.
2. VHOST_USER_GET_QUEUE_NUM returns the number of virtqueues, not the
   number of queue pairs.

Reword the section so that the vhost-user-net specific part is relegated
to the very end: we acknowledge that vhost-user-net historically
automatically enabled the first queue pair.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190626074815.19994-5-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/interop/vhost-user.rst

@@ -324,19 +324,20 @@ must support changing some configuration aspects on the fly.
 Multiple queue support
 ----------------------
 
-Multiple queue is treated as a protocol extension, hence the slave has
-to implement protocol features first. The multiple queues feature is
-supported only when the protocol feature ``VHOST_USER_PROTOCOL_F_MQ``
-(bit 0) is set.
+Multiple queue support allows the slave to advertise the maximum number of
+queues.  This is treated as a protocol extension, hence the slave has to
+implement protocol features first. The multiple queues feature is supported
+only when the protocol feature ``VHOST_USER_PROTOCOL_F_MQ`` (bit 0) is set.
 
-The max number of queue pairs the slave supports can be queried with
-message ``VHOST_USER_GET_QUEUE_NUM``. Master should stop when the
-number of requested queues is bigger than that.
+The max number of queues the slave supports can be queried with message
+``VHOST_USER_GET_QUEUE_NUM``. Master should stop when the number of requested
+queues is bigger than that.
 
 As all queues share one connection, the master uses a unique index for each
-queue in the sent message to identify a specified queue. One queue pair
-is enabled initially. More queues are enabled dynamically, by sending
-message ``VHOST_USER_SET_VRING_ENABLE``.
+queue in the sent message to identify a specified queue.
+
+The master enables queues by sending message ``VHOST_USER_SET_VRING_ENABLE``.
+vhost-user-net has historically automatically enabled the first queue pair.
 
 Migration
 ---------