docs(ngValue): add docs for ngValue directive

Closes #4267

src/ng/directive/input.js

@@ -1330,17 +1330,66 @@ var ngListDirective = function() {
 
 
 var CONSTANT_VALUE_REGEXP = /^(true|false|\d+)$/;
-
+/**
+ * @ngdoc directive
+ * @name ng.directive:ngValue
+ *
+ * @description
+ * Binds the given expression to the value of `input[select]` or `input[radio]`, so
+ * that when the element is selected, the `ngModel` of that element is set to the
+ * bound value.
+ *
+ * `ngValue` is useful when dynamically generating lists of radio buttons using `ng-repeat`, as
+ * shown below.
+ *
+ * @element input
+ * @param {string=} ngValue angular expression, whose value will be bound to the `value` attribute
+ *   of the `input` element
+ *
+ * @example
+    <doc:example>
+      <doc:source>
+       <script>
+          function Ctrl($scope) {
+            $scope.names = ['pizza', 'unicorns', 'robots'];
+            $scope.my = { favorite: 'unicorns' };
+          }
+       </script>
+        <form ng-controller="Ctrl">
+          <h2>Which is your favorite?</h2>
+            <label ng-repeat="name in names" for="{{name}}">
+              {{name}}
+              <input type="radio"
+                     ng-model="my.favorite"
+                     ng-value="name"
+                     id="{{name}}"
+                     name="favorite">
+            </label>
+          </span>
+          <div>You chose {{my.favorite}}</div>
+        </form>
+      </doc:source>
+      <doc:scenario>
+        it('should initialize to model', function() {
+          expect(binding('my.favorite')).toEqual('unicorns');
+        });
+        it('should bind the values to the inputs', function() {
+          input('my.favorite').select('pizza');
+          expect(binding('my.favorite')).toEqual('pizza');
+        });
+      </doc:scenario>
+    </doc:example>
+ */
 var ngValueDirective = function() {
   return {
     priority: 100,
     compile: function(tpl, tplAttr) {
       if (CONSTANT_VALUE_REGEXP.test(tplAttr.ngValue)) {
-        return function(scope, elm, attr) {
+        return function ngValueConstantLink(scope, elm, attr) {
           attr.$set('value', scope.$eval(attr.ngValue));
         };
       } else {
-        return function(scope, elm, attr) {
+        return function ngValueLink(scope, elm, attr) {
           scope.$watch(attr.ngValue, function valueWatchAction(value) {
             attr.$set('value', value);
           });