docs(input): document ngValue directive

Extend the example with ng-value showing how to deal with default checked radio boxes.

Closes #5654

src/ng/directive/input.js

@@ -309,6 +309,8 @@ var inputType = {
    * @param {string=} name Property name of the form under which the control is published.
    * @param {string=} ngChange Angular expression to be executed when input changes due to user
    *    interaction with the input element.
+   * @param {string} ngValue Angular expression which sets the value to which the expression should
+   *    be set when selected.
    *
    * @example
       <doc:example>
@@ -316,21 +318,26 @@ var inputType = {
          <script>
            function Ctrl($scope) {
              $scope.color = 'blue';
+             $scope.specialValue = {
+               "id": "12345",
+               "value": "green"
+             };
            }
          </script>
          <form name="myForm" ng-controller="Ctrl">
            <input type="radio" ng-model="color" value="red">  Red <br/>
-           <input type="radio" ng-model="color" value="green"> Green <br/>
+           <input type="radio" ng-model="color" ng-value="specialValue"> Green <br/>
            <input type="radio" ng-model="color" value="blue"> Blue <br/>
-           <tt>color = {{color}}</tt><br/>
+           <tt>color = {{color | json}}</tt><br/>
           </form>
+          Note that `ng-value="specialValue"` sets radio item's value to be the value of `$scope.specialValue`.
         </doc:source>
         <doc:scenario>
           it('should change state', function() {
-            expect(binding('color')).toEqual('blue');
+            expect(binding('color')).toEqual('"blue"');
 
             input('color').select('red');
-            expect(binding('color')).toEqual('red');
+            expect(binding('color')).toEqual('"red"');
           });
         </doc:scenario>
       </doc:example>