docs: improve DefaultValueAccessor directive docs (#39404)

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

Closes #35375.

PR Close #39404

packages/forms/src/directives/default_value_accessor.ts

@@ -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