docs: vhost-user: rewrite section on ring state machine

This section is using the word "back-end" to refer to the
"slave's back-end", and talking about the "client" for
what the rest of the document calls the "slave".

Rework it to free the use of the term "back-end", which in
the next patch will replace "slave".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210226143413.188046-3-pbonzini@redhat.com>
Message-Id: <20220321153037.3622127-6-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/interop/vhost-user.rst

@@ -332,40 +332,36 @@ bit was dedicated for this purpose::
 
   #define VHOST_USER_F_PROTOCOL_FEATURES 30
 
-Starting and stopping rings
----------------------------
+Ring states
+-----------
 
-Client must only process each ring when it is started.
+Rings can be in one of three states:
 
-Client must only pass data between the ring and the backend, when the
-ring is enabled.
+* stopped: the slave must not process the ring at all.
 
-If ring is started but disabled, client must process the ring without
-talking to the backend.
+* started but disabled: the slave must process the ring without
+  causing any side effects.  For example, for a networking device,
+  in the disabled state the slave must not supply any new RX packets,
+  but must process and discard any TX packets.
 
-For example, for a networking device, in the disabled state client
-must not supply any new RX packets, but must process and discard any
-TX packets.
+* started and enabled.
 
-If ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been negotiated, the
-ring is initialized in an enabled state.
+Each ring is initialized in a stopped state.  The slave must start
+ring upon receiving a kick (that is, detecting that file descriptor is
+readable) on the descriptor specified by ``VHOST_USER_SET_VRING_KICK``
+or receiving the in-band message ``VHOST_USER_VRING_KICK`` if negotiated,
+and stop ring upon receiving ``VHOST_USER_GET_VRING_BASE``.
 
-If ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, the ring is
-initialized in a disabled state. Client must not pass data to/from the
-backend until ring is enabled by ``VHOST_USER_SET_VRING_ENABLE`` with
-parameter 1, or after it has been disabled by
-``VHOST_USER_SET_VRING_ENABLE`` with parameter 0.
+Rings can be enabled or disabled by ``VHOST_USER_SET_VRING_ENABLE``.
 
-Each ring is initialized in a stopped state, client must not process
-it until ring is started, or after it has been stopped.
+If ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been negotiated, the
+ring starts directly in the enabled state.
 
-Client must start ring upon receiving a kick (that is, detecting that
-file descriptor is readable) on the descriptor specified by
-``VHOST_USER_SET_VRING_KICK`` or receiving the in-band message
-``VHOST_USER_VRING_KICK`` if negotiated, and stop ring upon receiving
-``VHOST_USER_GET_VRING_BASE``.
+If ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, the ring is
+initialized in a disabled state and is enabled by
+``VHOST_USER_SET_VRING_ENABLE`` with parameter 1.
 
-While processing the rings (whether they are enabled or not), client
+While processing the rings (whether they are enabled or not), the slave
 must support changing some configuration aspects on the fly.
 
 Multiple queue support