Add explanation of setAppElement in settings

Author: interactivellama

components/settings.js

@@ -24,12 +24,24 @@ module.exports = {
 			assetsPath = path;
 		}
 	},
+
 	getAssetsPath: () => String(assetsPath),
+
+	/*
+	 * The app element allows you to specify the portion of your app that should be hidden (via aria-hidden)
+	to prevent assistive technologies such as screenreaders from reading content outside of the content of
+	your modal.  It can be specified in the following ways:
+	 * element
+	Modal.setAppElement(appElement);
+	 * query selector - uses the first element found if you pass in a class.
+	Modal.setAppElement('#your-app-element');
+	*/
 	setAppElement: (el) => {
 		if (el) {
 			appRoot = el;
 			Modal.setAppElement(el);
 		}
 	},
+
 	getAppElement: () => appRoot
 };