Allow UNIX socket option for VNC websocket

- Remove unix socket option limitation for VNC websocket
- Reflect websocket option changes in documentation

Signed-off-by: Sergii Zasenko <sergii@zasenko.name>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230724100353.16628-1-sergii@zasenko.name>

qemu-options.hx

@@ -2516,6 +2516,10 @@ SRST
         host. It is possible to control the websocket listen address
         independently, using the syntax ``websocket``\ =host:port.
 
+        Websocket could be allowed over UNIX domain socket, using the syntax
+        ``websocket``\ =unix:path, where path is the location of a unix socket
+        to listen for connections on.
+
         If no TLS credentials are provided, the websocket connection
         runs in unencrypted mode. If TLS credentials are provided, the
         websocket connection requires encrypted client connections.