Add comments to explain checking for userAgent

lib/index.js

@@ -5,8 +5,10 @@
 'use strict';
 
 if (typeof Realm != 'undefined') {
+    // The global Realm constructor should be available on device (using JavaScriptCore).
     module.exports = Realm;  // eslint-disable-line no-undef
 } else if (navigator.userAgent) {
+    // The userAgent will be defined when running in a browser (such as Chrome debugging mode).
     module.exports = require('./realm');
 } else {
     throw new Error('Missing Realm constructor - please ensure RealmReact framework is included!');