docs(guide/Working With CSS): ng-touched/untouched

Add description for ng-touched and ng-untouched CSS classes.

Closes #10302

Author: philippd

docs/content/guide/css-styling.ngdoc

@@ -28,6 +28,11 @@ Angular sets these CSS classes. It is up to your application to provide useful s
   - **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`.
+    
+* `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
+    the class is changed to `ng-touched`.
 
 
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