docs(guide/css-styling): form controls are not always input elements

People frequently write custom form controls using the `ngModel` directive, this just
refactors the text to be more clear that this is possible (imho).

Author: caitp

docs/content/guide/css-styling.ngdoc

@@ -21,17 +21,17 @@ Angular sets these CSS classes. It is up to your application to provide useful s
     `{{}}` curly braces, for example. (see {@link guide/databinding databinding} guide)
 
 * `ng-invalid`, `ng-valid`
-  - **Usage:** angular applies this class to an input widget element if that element's input does
+  - **Usage:** angular applies this class to a form control widget element if that element's input does
     not pass validation. (see {@link ng.directive:input input} directive)
 
 * `ng-pristine`, `ng-dirty`
-  - **Usage:** angular {@link ng.directive:input input} directive applies `ng-pristine` class
-    to a new input widget element which did not have user interaction. Once the user interacts with
-    the input widget the class is changed to `ng-dirty`.
+  - **Usage:** angular {@link ng.directive:ngModel ngModel} directive applies `ng-pristine` class
+    to a new form control widget which did not have user interaction. Once the user interacts with
+    the form control, the class is changed to `ng-dirty`.
 
 * `ng-touched`, `ng-untouched`
   - **Usage:** angular {@link ng.directive:ngModel ngModel} directive applies `ng-untouched` class
-    to a new input widget element which has not been blurred. Once the user blurs the input widget
+    to a new form control widget which has not been blurred. Once the user blurs the form control,
     the class is changed to `ng-touched`.