docs(ionCheckbox): write documentation

js/ext/angular/src/directive/ionicRadio.js

@@ -5,6 +5,23 @@ angular.module('ionic.ui.radio', [])
 
 // The radio button is a radio powered element with only
 // one possible selection in a set of options.
+/**
+ * @ngdoc directive
+ * @name ionRadio
+ * @module ionic
+ * @restrict E
+ * @description
+ * No different than the HTML radio input, except it's styled differently.
+ *
+ * Behaves like any [AngularJS radio](http://docs.angularjs.org/api/ng/input/input[radio]).
+ *
+ * @usage
+ * ```html
+ * <ion-radio ng-model="choice" value="A">Choose A</ion-radio>
+ * <ion-radio ng-model="choice" value="B">Choose B</ion-radio>
+ * <ion-radio ng-model="choice" value="C">Choose C</ion-radio>
+ * ```
+ */
 .directive('ionRadio', function() {
   return {
     restrict: 'E',