message: docs: improve DefaultValueAccessor directive docs
  This commit improves the DefaultValueAccessor directive docs by:
  adding the `ngDefaultControl` as a search keyword to the description
  adding an example of the `ngDefaultControl` usage
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  PR Close
diff: @@ -36,6 +36,9 @@ export const COMPOSITION_BUFFER_MODE = new InjectionToken<boolean>('CompositionE
  /**
  * @description
  + *
  + * {@searchKeywords ngDefaultControl}
  + *
  * The default `ControlValueAccessor` for writing a value and listening to changes on input
  * elements. The accessor is used by the `FormControlDirective`, `FormControlName`, and
  * `NgModel` directives.
  @@ -55,6 +58,15 @@ export const COMPOSITION_BUFFER_MODE = new InjectionToken<boolean>('CompositionE
  * <input type="text" [formControl]="firstNameControl">
  * ```
  *
  + * This value accessor is used by default for `<input type="text">` and `<textarea>` elements, but
  + * you could also use it for custom components that have similar behavior and do not require special
  + * processing. In order to attach the default value accessor to a custom element, add the
  + * `ngDefaultControl` attribute as shown below.
  + *
  + * ```
  + * <custom-input-component ngDefaultControl [(ngModel)]="value"></custom-input-component>
  + * ```
  + *
  * @ngModule ReactiveFormsModule
  * @ngModule FormsModule
  * @publicApi
  