docs: add document for `afterClose` props for Modal (#4448)

+ close #4445

components/modal/index.en-US.md

@@ -31,6 +31,7 @@ and so on.
 | maskClosable | Determine whether to close the modal dialog when clicked mask of it. | Boolean   | true       |
 | style | Style of floating layer, typically used at least for adjusting the position. | Object   | - |
 | 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 | - |
 
 ### Modal.xxx()
 

components/modal/index.zh-CN.md

@@ -30,6 +30,7 @@ title: Modal
 | maskClosable | 点击蒙层是否允许关闭 | Boolean   | true       |
 | style | 可用于设置浮层的样式，调整浮层位置等 | Object   | - |
 | wrapClassName | 对话框外层容器的类名 | String   | - |
+| afterClose | Modal 完全关闭后的回调 | function | 无 |
 
 ### Modal.xxx()