Add documentations

lib/src/fluwx.dart

@@ -91,6 +91,7 @@ class Fluwx {
     return FluwxPlatform.instance.getExtMsg();
   }
 
+  /// Pay with WeChat. See [PayType]
   Future<bool> pay({required PayType which}) async {
     return FluwxPlatform.instance.pay(which);
   }

lib/src/foundation/cancelable.dart

+/*
+ * Copyright (c) 2023.  OpenFlutter Project
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more contributor
+ * license agreements.  See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.  The ASF licenses this
+ * file to you under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License.  You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
 import '../response/wechat_response.dart';
 
 typedef WeChatResponseSubscriber = Function(WeChatResponse response);

lib/src/foundation/open_type.dart

@@ -15,6 +15,7 @@ class Browser extends OpenType {
   Map<String, dynamic> get arguments => {'url': url};
 }
 
+/// Rank list
 class RankList extends OpenType {}
 
 /// see * https://pay.weixin.qq.com/wiki/doc/apiv3_partner/Offline/apis/chapter6_2_1.shtml