[core] add extendApp support - added description/link

lib/firebase-app.js

@@ -81,6 +81,10 @@ export default class FirebaseApp {
   }
 
   /**
+   * Undocumented firebase web sdk method that allows adding additional properties onto
+   * a firebase app instance.
+   *
+   * See: https://github.com/firebase/firebase-js-sdk/blob/master/tests/app/firebase_app.test.ts#L328
    *
    * @param props
    */