docs(forms): migrate deprecated @howToUse tags (#23186)

In this case they have been converted to `@description` tags.

PR Close #23186

packages/forms/src/directives/ng_form.ts

@@ -29,7 +29,7 @@ const resolvedPromise = Promise.resolve(null);
  * @whatItDoes Creates a top-level {@link FormGroup} instance and binds it to a form
  * to track aggregate form value and validation status.
  *
- * @howToUse
+ * @description
  *
  * As soon as you import the `FormsModule`, this directive becomes active by default on
  * all `<form>` tags.  You don't need to add a special selector.