Add to 'supported' API example (so it does something with the result) (…

…#9989)

* add to example so it does something with the result

* use multi-line code formatting

* add closing }

Co-authored-by: Heather Stenson <heather.stenson@mapbox.com>

src/index.js

@@ -193,7 +193,10 @@ Debug.extend(exported, {isSafari, getPerformanceMetrics: PerformanceUtils.getPer
  *   be dramatically worse than expected (e.g. a software WebGL renderer would be used).
  * @return {boolean}
  * @example
- * mapboxgl.supported() // = true
+ * // Show an alert if the browser does not support Mapbox GL
+ * if (!mapboxgl.supported()) {
+ *   alert('Your browser does not support Mapbox GL');
+ * }
  * @see [Check for browser support](https://www.mapbox.com/mapbox-gl-js/example/check-for-support/)
  */