docs(forms): add example apps for ngModelGroup (#11525)

modules/@angular/forms/src/directives/ng_model_group.ts

@@ -21,39 +21,27 @@ export const modelGroupProvider: any = {
 };
 
 /**
- * Creates and binds a model group to a DOM element.
+ * @whatItDoes Creates and binds a {@link FormGroup} instance to a DOM element.
  *
- * This directive can only be used as a child of {@link NgForm}.
+ * @howToUse
  *
- * ```typescript
- * @Component({
- *   selector: 'my-app',
- *   template: `
- *     <div>
- *       <h2>Angular forms Example</h2>
- *       <form #f="ngForm">
- *         <div ngModelGroup="name" #mgName="ngModelGroup">
- *           <h3>Enter your name:</h3>
- *           <p>First: <input name="first" ngModel required></p>
- *           <p>Middle: <input name="middle" ngModel></p>
- *           <p>Last: <input name="last" ngModel required></p>
- *         </div>
- *         <h3>Name value:</h3>
- *         <pre>{{ mgName.value | json }}</pre>
- *         <p>Name is {{mgName?.valid ? "valid" : "invalid"}}</p>
- *         <h3>What's your favorite food?</h3>
- *         <p><input name="food" ngModel></p>
- *         <h3>Form value</h3>
- *         <pre>{{ f.value | json }}</pre>
- *       </form>
- *     </div>
- *   `
- * })
- * export class App {}
- * ```
+ * This directive can only be used as a child of {@link NgForm} (or in other words,
+ * within `<form>` tags).
  *
- * This example declares a model group for a user's name. The value and validation state of
- * this group can be accessed separately from the overall form.
+ * Use this directive if you'd like to create a sub-group within a form. This can
+ * come in handy if you want to validate a sub-group of your form separately from
+ * the rest of your form, or if some values in your domain model make more sense to
+ * consume together in a nested object.
+ *
+ * Pass in the name you'd like this sub-group to have and it will become the key
+ * for the sub-group in the form's full value. You can also export the directive into
+ * a local template variable using `ngModelGroup` (ex: `#myGroup="ngModelGroup"`).
+ *
+ * {@example forms/ts/ngModelGroup/ng_model_group_example.ts region='Component'}
+ *
+ * * **npm package**: `@angular/forms`
+ *
+ * * **NgModule**: `FormsModule`
  *
  * @stable
  */