docs(ngModelController): improve $rollbackViewValue description & example

The example has been expanded to make it easier to provoke the
behavior that the description is talking about (rollbackViewValue
and programmatic model updates)

Related #13340

Author: Narretz

src/ng/directive/ngModel.js

@@ -435,11 +435,14 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    * which may be caused by a pending debounced event or because the input is waiting for a some
    * future event.
    *
-   * If you have an input that uses `ng-model-options` to set up debounced events or events such
-   * as blur you can have a situation where there is a period when the `$viewValue`
-   * is out of synch with the ngModel's `$modelValue`.
+   * If you have an input that uses `ng-model-options` to set up debounced updates or updates that
+   * depend on special events such as blur, you can have a situation where there is a period when
+   * the `$viewValue` is out of sync with the ngModel's `$modelValue`.
    *
-   * In this case, you can run into difficulties if you try to update the ngModel's `$modelValue`
+   * In this case, you can use `$rollbackViewValue()` to manually cancel the debounced / future update
+   * and reset the input to the last committed view value.
+   *
+   * It is also possible that you run into difficulties if you try to update the ngModel's `$modelValue`
    * programmatically before these debounced/future events have resolved/occurred, because Angular's
    * dirty checking mechanism is not able to tell whether the model has actually changed or not.
    *
@@ -452,39 +455,63 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    *     angular.module('cancel-update-example', [])
    *
    *     .controller('CancelUpdateController', ['$scope', function($scope) {
-   *       $scope.resetWithCancel = function(e) {
-   *         if (e.keyCode == 27) {
-   *           $scope.myForm.myInput1.$rollbackViewValue();
-   *           $scope.myValue = '';
-   *         }
-   *       };
-   *       $scope.resetWithoutCancel = function(e) {
+   *       $scope.model = {};
+   *
+   *       $scope.setEmpty = function(e, value, rollback) {
    *         if (e.keyCode == 27) {
-   *           $scope.myValue = '';
+   *           e.preventDefault();
+   *           if (rollback) {
+   *             $scope.myForm[value].$rollbackViewValue();
+   *           }
+   *           $scope.model[value] = '';
    *         }
    *       };
    *     }]);
    *   </file>
    *   <file name="index.html">
    *     <div ng-controller="CancelUpdateController">
-   *       <p>Try typing something in each input.  See that the model only updates when you
-   *          blur off the input.
-   *        </p>
-   *        <p>Now see what happens if you start typing then press the Escape key</p>
+   *        <p>Both of these inputs are only updated if they are blurred. Hitting escape should
+   *        empty them. Follow these steps and observe the difference:</p>
+   *       <ol>
+   *         <li>Type something in the input. You will see that the model is not yet updated</li>
+   *         <li>Press the Escape key.
+   *           <ol>
+   *             <li> In the first example, nothing happens, because the model is already '', and no
+   *             update is detected. If you blur the input, the model will be set to the current view.
+   *             </li>
+   *             <li> In the second example, the pending update is cancelled, and the input is set back
+   *             to the last committed view value (''). Blurring the input does nothing.
+   *             </li>
+   *           </ol>
+   *         </li>
+   *       </ol>
    *
    *       <form name="myForm" ng-model-options="{ updateOn: 'blur' }">
-   *         <p id="inputDescription1">With $rollbackViewValue()</p>
-   *         <input name="myInput1" aria-describedby="inputDescription1" ng-model="myValue"
-   *                ng-keydown="resetWithCancel($event)"><br/>
-   *         myValue: "{{ myValue }}"
+   *         <div>
+   *        <p id="inputDescription1">Without $rollbackViewValue():</p>
+   *         <input name="value1" aria-describedby="inputDescription1" ng-model="model.value1"
+   *                ng-keydown="setEmpty($event, 'value1')">
+   *         value1: "{{ model.value1 }}"
+   *         </div>
    *
-   *         <p id="inputDescription2">Without $rollbackViewValue()</p>
-   *         <input name="myInput2" aria-describedby="inputDescription2" ng-model="myValue"
-   *                ng-keydown="resetWithoutCancel($event)"><br/>
-   *         myValue: "{{ myValue }}"
+   *         <div>
+   *        <p id="inputDescription2">With $rollbackViewValue():</p>
+   *         <input name="value2" aria-describedby="inputDescription2" ng-model="model.value2"
+   *                ng-keydown="setEmpty($event, 'value2', true)">
+   *         value2: "{{ model.value2 }}"
+   *         </div>
    *       </form>
    *     </div>
    *   </file>
+       <file name="style.css">
+          div {
+            display: table-cell;
+          }
+          div:nth-child(1) {
+            padding-right: 30px;
+          }
+
+        </file>
    * </example>
    */
   this.$rollbackViewValue = function() {