Updated v2_0_cpp_adaptor (markdown)

v2_0_cpp_adaptor.md

@@ -384,6 +384,12 @@ class my_class {
 };
 ```
 
+Before the version 4.0.3, you need to declare for object adaptor as follows:
+```C++
+    template <typename T, typename Enabler, typename Enabler2>
+    friend struct msgpack::MSGPACK_DEFAULT_API_NS::adaptor::object;
+```
+
 You need to declare only the class template specializations that you defined.