Add allowSwipeDismissal documentation (facebook#4658)

* Update moda docs

* Apply suggestions from code review

Author: okwasniewski

docs/modal.md

@@ -176,10 +176,21 @@ The `onOrientationChange` callback is called when the orientation changes while
 
 ---
 
+### `allowSwipeDismissal` <div class="label ios">iOS</div>
+
+Controls whether the modal can be dismissed by swiping down on iOS.
+This requires you to implement the `onRequestClose` prop to handle the dismissal.
+
+| Type | Default |
+| ---- | ------- |
+| bool | `false` |
+
+---
+
 ### `onRequestClose`
 
 The `onRequestClose` callback is called when the user taps the hardware back button on Android or the menu button on Apple TV. Because of this required prop, be aware that `BackHandler` events will not be emitted as long as the modal is open.
-On iOS, this callback is called when a Modal is being dismissed using a drag gesture when `presentationStyle` is `pageSheet or formSheet`
+On iOS, this callback is called when a Modal is being dismissed using a drag gesture when `presentationStyle` is `pageSheet or formSheet`. When `allowSwipeDismissal` is enabled this callback will be called after dismissing the modal.
 
 | Type                                                                                                                                                                                           |
 | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |