doc: add more websocket examples

doc/example-config-files/multidomain/config.js.multidomain.example

         focus: 'focus.jitsi.example.com',
     },
     useNicks: false,
-    bosh: '//jitsi.example.com/http-bind' // FIXME: use xep-0156 for that
+    bosh: '//jitsi.example.com/http-bind', // FIXME: use xep-0156 for that
+    websocket: 'wss://jitsi.example.com/xmpp-websocket'
 };

doc/example-config-files/multidomain/jitsi.example.com.multidomain.example

         rewrite ^/(.*)$ /http-bind;
     }
 
+    # websockets for subdomains
+    location ~ ^/([^/?&:'"]+)/xmpp-websocket {
+        set $subdomain "$1.";
+        set $subdir "$1/";
+        set $prefix "$1";
+
+        rewrite ^/(.*)$ /xmpp-websocket;
+    }
 }