Add doc about clearing Modal old state

components/modal/index.en-US.md

@@ -33,6 +33,15 @@ and so on.
 | wrapClassName | The class name of the container of the modal dialog | String   | - |
 | afterClose | Specify a function that will be called when modal is closed completely. | function | - |
 
+#### Destroy on close
+
+> The state of Modal will be preserved at it's component lifecircle.
+> If you wish to open it with brand new state everytime, you need to reset state manually. Or simply [give a new random key](https://github.com/ant-design/ant-design/issues/4165) to Modal when visible is changed to `true`, React will treat it as a new component.
+
+> ```
+> <Modal key={this.state.newRandomKey} visible={this.state.visible} />
+> ```
+
 ### Modal.xxx()
 
 There are five ways to display the information based on the content's nature:

components/modal/index.zh-CN.md

@@ -32,6 +32,15 @@ title: Modal
 | wrapClassName | 对话框外层容器的类名 | String   | - |
 | afterClose | Modal 完全关闭后的回调 | function | 无 |
 
+#### 清空旧数据
+
+> `<Modal />` 组件有标准的 React 生命周期，关闭后状态不会自动清空。
+> 如果希望每次打开都是新内容，需要自行手动清空旧的状态。或者打开时给 Modal 设置一个[全新的 key](https://github.com/ant-design/ant-design/issues/4165)， React 会渲染出一个全新的对话框。
+
+> ```
+> <Modal key={this.state.newRandomKey} visible={this.state.visible} />
+> ```
+
 ### Modal.xxx()
 
 包括：