docs(remote_config): update web support documentation for onConfigUpdated

Author: SelaseKay

packages/firebase_remote_config/firebase_remote_config/lib/src/firebase_remote_config.dart

@@ -160,7 +160,8 @@ class FirebaseRemoteConfig extends FirebasePluginPlatform {
   /// Starts listening for real-time config updates from the Remote Config backend and automatically
   /// fetches updates from the RC backend when they are available.
   ///
-  /// This feature is not supported on Web.
+  /// On web, you must call [fetchAndActivate] before listening to this stream. Events will only be
+  /// received after an initial call to [fetchAndActivate].
   ///
   /// If a connection to the Remote Config backend is not already open, calling this method will
   /// open it. Multiple listeners can be added by calling this method again, but subsequent calls

tests/integration_test/firebase_remote_config/firebase_remote_config_e2e_test.dart

@@ -138,8 +138,6 @@ void main() {
 
           await configSubscription.cancel();
         },
-        // This feature is not supported on Web
-        skip: kIsWeb,
       );
 
       test('default values', () async {