Add documentation for onMomentumScrollEnd

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Closes https://github.com/facebook/react-native/pull/15144

Differential Revision: D5478574

Pulled By: shergin

fbshipit-source-id: 33c49f0efdfb3a518e1ee254b1dc01ec22f09269

Libraries/Components/ScrollView/ScrollView.js

@@ -235,6 +235,10 @@ const ScrollView = createReactClass({
      * @platform ios
      */
     minimumZoomScale: PropTypes.number,
+    /**
+     * Called when the momentum scroll ends (scroll which occurs as the ScrollView glides to a stop).
+     */
+    onMomentumScrollEnd: PropTypes.func,
     /**
      * Fires at most once per frame during scrolling. The frequency of the
      * events can be controlled using the `scrollEventThrottle` prop.