Bug 1533679 - Document --remote-debugger-port in usage message. r=ato

Differential Revision: https://phabricator.services.mozilla.com/D22684

--HG--
extra : moz-landing-system : lando

remote/RemoteAgent.js

@@ -179,7 +179,8 @@ class ParentRemoteAgent {
   }
 
   get helpInfo() {
-    return "  --remote-debugger [<host>][:<port>] Start the Firefox remote agent, which is \n" +
+    return "  --remote-debugger [<host>][:<port>]\n" +
+           "  --remote-debugging-port <port> Start the Firefox remote agent, which is \n" +
            "                     a low-level debugging interface based on the CDP protocol.\n" +
            "                     Defaults to listen on localhost:9222.\n";
   }