Update SegmentedControlIOS.ios.js

Summary:Updating the comments to clarify that the selectedIndex prop is not just for pre-selecting an index, but that it can also be used to programmatically change the value of the selected index.
Closes https://github.com/facebook/react-native/pull/6519

Differential Revision: D3075199

Pulled By: mkonicek

fb-gh-sync-id: 8ce336ad491bb2fed13df15ebddb1b24535c50ff
shipit-source-id: 8ce336ad491bb2fed13df15ebddb1b24535c50ff

Libraries/Components/SegmentedControlIOS/SegmentedControlIOS.ios.js

@@ -31,6 +31,23 @@ type Event = Object;
 
 /**
  * Use `SegmentedControlIOS` to render a UISegmentedControl iOS.
+ *
+ * #### Programmatically changing selected index
+ *
+ * The selected index can be changed on the fly by assigning the
+ * selectIndex prop to a state variable, then changing that variable.
+ * Note that the state variable would need to be updated as the user
+ * selects a value and changes the index, as shown in the example below.
+ *
+ * ````
+ * <SegmentedControlIOS
+ *   values={['One', 'Two']}
+ *   selectedIndex={this.state.selectedIndex}
+ *   onChange={(event) => {
+ *     this.setState({selectedIndex: event.nativeEvent.selectedSegmentIndex});
+ *   }}
+ * />
+ * ````
  */
 var SegmentedControlIOS = React.createClass({
   mixins: [NativeMethodsMixin],
@@ -43,7 +60,7 @@ var SegmentedControlIOS = React.createClass({
     values: PropTypes.arrayOf(PropTypes.string),
 
     /**
-     * The index in `props.values` of the segment to be pre-selected
+     * The index in `props.values` of the segment to be (pre)selected.  
      */
     selectedIndex: PropTypes.number,