update Modal.js to fix #10662

Summary:
further discussion: should there be a `onClose` or `onClosed` to pair with `onShow`? which would make a workaround for #10471 much easier
Closes https://github.com/facebook/react-native/pull/10669

Differential Revision: D4133832

Pulled By: hramos

fbshipit-source-id: 644a5bb6b9da697c81fc96ae4da196ba5b4050cb

Libraries/Modal/Modal.js

@@ -98,9 +98,8 @@ class Modal extends React.Component {
      */
     visible: PropTypes.bool,
     /**
-     * The `onRequestClose` prop allows passing a function that will be called once the modal has been dismissed.
-     *
-     * _On the Android platform, this is a required function._
+     * The `onRequestClose` callback is called when the user taps the hardware back button.
+     * @platform android
      */
     onRequestClose: Platform.OS === 'android' ? PropTypes.func.isRequired : PropTypes.func,
     /**