docs: Document customized form control

Related: #5700

components/form/demo/customized-form-controls.md

@@ -10,13 +10,14 @@ title:
 自定义或第三方的表单控件，也可以与 Form 组件一起使用。只要该组件遵循以下的约定：
 > * 提供受控属性 `value` 或其它与 [`valuePropName`](http://ant.design/components/form/#getFieldDecorator-参数) 的值同名的属性。
 > * 提供 `onChange` 事件或 [`trigger`](http://ant.design/components/form/#getFieldDecorator-参数) 的值同名的事件。
+> * 不能是函数式组件。
 
 ## en-US
 
 Customized or third-party form controls can be used in Form, too. Controls must follow these conventions:
 > * It has a controlled property `value` or other name which is equal to the value of [`valuePropName`](http://ant.design/components/form/?locale=en-US#getFieldDecorator's-parameters).
 > * It has event `onChange` or an event which name is equal to the value of [`trigger`](http://ant.design/components/form/?locale=en-US#getFieldDecorator's-parameters).
-
+> * It must be a class component.
 
 ````jsx
 import { Form, Input, Select, Button } from 'antd';