Add doc for RefreshControl on the website

Summary:
Exposes the doc for RefreshControl and add a link to the component page in ScrollView.
Closes https://github.com/facebook/react-native/pull/5209

Reviewed By: svcscm

Differential Revision: D2818217

Pulled By: mkonicek

fb-gh-sync-id: eb1ae70103e6a02af3a12866509f68eacc413dfd

Libraries/Components/ScrollView/ScrollView.js

@@ -297,6 +297,8 @@ var ScrollView = React.createClass({
     /**
      * A RefreshControl component, used to provide pull-to-refresh
      * functionality for the ScrollView.
+     *
+     * See [RefreshControl](http://facebook.github.io/react-native/docs/refreshcontrol.html).
      */
     refreshControl: PropTypes.element,