docs(forms): migrate @whatItDoes tags to the description (#23186)

We get the overview for the doc by splitting off the first
paragraph.

PR Close #23186

packages/forms/src/directives/ng_form.ts

@@ -26,11 +26,11 @@ export const formDirectiveProvider: any = {
 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.
- *
  * @description
  *
+ * Creates a top-level {@link FormGroup} instance and binds it to a form
+ * to track aggregate form value and validation status.
+ *
  * 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.
  *