Document arguments to onSelectionChange

Summary:
To update the documentation. Works on iOS, so I assume this is the same on Android.
Closes https://github.com/facebook/react-native/pull/11575

Differential Revision: D4494464

Pulled By: mkonicek

fbshipit-source-id: c7480d39ed0849401efaa080948c14fa0cb7a08d

Libraries/Components/TextInput/TextInput.js

@@ -367,6 +367,8 @@ const TextInput = React.createClass({
     onEndEditing: PropTypes.func,
     /**
      * Callback that is called when the text input selection is changed.
+     * This will be called with
+     * `{ nativeEvent: { selection: { start, end } } }`.
      */
     onSelectionChange: PropTypes.func,
     /**