Add documentations

lib/src/foundation/auto_deduct.dart

 part of 'arguments.dart';
 
+/// Auto Deduct
 class AutoDeduct with _Argument {
   final Map<String, String> queryInfo;
   final int businessType;

lib/src/foundation/cancelable.dart

@@ -19,6 +19,7 @@
 
 import 'package:flutter/foundation.dart';
 
+/// A mixin that provides a method to cancel an operation.
 mixin FluwxCancelable {
   void cancel();
 }

lib/src/foundation/open_type.dart

 part of 'arguments.dart';
 
+/// The target you want to open.
 sealed class OpenType with _Argument {}
 
 /// Just open WeChat app.
@@ -54,6 +55,7 @@ class Invoice extends OpenType {
       };
 }
 
+/// Customer service chat, known as "微信客服" in WeChat
 class CustomerServiceChat extends OpenType {
   final String corpId;
   final String url;