Document subtitle parameter for screen push (#2735)

docs/screen-api.md

 this.props.navigator.push({
   screen: 'example.ScreenThree', // unique ID registered with Navigation.registerScreen
   title: undefined, // navigation bar title of the pushed screen (optional)
+  subtitle: undefined, // navigation bar subtitle of the pushed screen (optional)
   titleImage: require('../../img/my_image.png'), // iOS only. navigation bar title image instead of the title text of the pushed screen (optional)
   passProps: {}, // Object that will be passed as props to the pushed screen (optional)
   animated: true, // does the push have transition animation or does it happen immediately (optional)
 
 You can define actions and listen for interactions on the pushed screen with the `PreviewActionPress` event.
 
-Previewed screens will have the prop `isPreview` that can be used to render different things when the screen is in the "Peek" state and will then recieve a navigator event of `willCommitPreview` when in the "Pop" state.
+Previewed screens will have the prop `isPreview` that can be used to render different things when the screen is in the "Peek" state and will then recieve a navigator event of `willCommitPreview` when in the "Pop" state.