docs(ngModel): add link to NgModelController

src/ng/directive/input.js

@@ -1109,7 +1109,8 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
  *
  * @description
  * Is a directive that tells Angular to do two-way data binding. It works together with `input`,
- * `select`, `textarea`. You can easily write your own directives to use `ngModel` as well.
+ * `select`, `textarea` and even custom form controls that use {@link ng.directive:ngModel.NgModelController
+ * NgModelController} exposed by this directive.
  *
  * `ngModel` is responsible for:
  *