Updated TouchableHighlight docs

Summary: ``onlyChild`` restriction is implemented in ``TouchableHighlight``, not ``TouchableOpacity``.
Closes https://github.com/facebook/react-native/pull/2542

Reviewed By: @​svcscm

Differential Revision: D2471539

Pulled By: @vjeux

Libraries/Components/Touchable/TouchableHighlight.js

@@ -57,6 +57,9 @@ var DEFAULT_PROPS = {
  *   );
  * },
  * ```
+ * > **NOTE**: TouchableHighlight supports only one child
+ * >
+ * > If you wish to have to have several child components, wrap them in a View.
  */
 
 var TouchableHighlight = React.createClass({