docs(en): Add firewall settings to a syncplay module

content/en/docs/Theory/selfprivacy_modules/_index.md

@@ -420,6 +420,9 @@ in
         ++ lib.optional (!cfg.enableChat) [ "--disable-chat" ];
     };
 
+    # We need to open a port for Syncplay to work.
+    networking.firewall.allowedTCPPorts = [ 8999 ];
+
     # Now we need to define a systemd slice and put syncplay service there.
     # It is required to track module's resource consumption.
     systemd = {