doc(Modal): Update document.

Author: jaywcjlove

packages/core/src/Modal/README.md

@@ -48,4 +48,11 @@ export default class ButtonGroupView extends Component {
 
 ## Props
 
-继承原生 Modal 属性 [`ModalProps`](https://facebook.github.io/react-native/docs/modal.html#props)
+继承原生 Modal 属性 [`ModalProps`](https://facebook.github.io/react-native/docs/modal.html#props)
+
+```ts
+export interface ModalProps extends MaskLayerProps {
+  placement?: 'top' | 'right' | 'bottom' | 'left';
+  onClosed?: () => void;
+}
+```