uac: extend docs for uac_auth() example, only t_relay() on success

- extend docs for uac_auth() example, only t_relay() on success
- otherwise you will get a 'no branches for forwarding' error from tm

(cherry picked from commit 668402e)

src/modules/uac/doc/uac_admin.xml

@@ -873,9 +873,9 @@ failure_route[TRUNKAUTH] {
         $avp(auser) = "test";
         $avp(apass) = "test";
         # $avp(apass) = "36d0a02793542b4961e8348347236dbf";
-        uac_auth();
-        # uac_auth("1");
-        t_relay();
+        if (uac_auth()) {
+            t_relay();
+        }
         exit;
     }
 }