docs(forms): update and re-organize validation guide

packages/forms/src/directives/ng_control_status.ts

@@ -38,7 +38,16 @@ export const ngControlStatusHost = {
 
 /**
  * Directive automatically applied to Angular form controls that sets CSS classes
- * based on control status (valid/invalid/dirty/etc).
+ * based on control status. The following classes are applied as the properties
+ * become true:
+ *
+ * * ng-valid
+ * * ng-invalid
+ * * ng-pending
+ * * ng-pristine
+ * * ng-dirty
+ * * ng-untouched
+ * * ng-touched
  *
  * @stable
  */