docs(forms): update description for FormBuilder.group (angular#26970)

PR Close angular#26970

Author: kara

packages/forms/src/form_builder.ts

@@ -32,7 +32,16 @@ export class FormBuilder {
    * @param controlsConfig A collection of child controls. The key for each child is the name
    * under which it is registered.
    *
-   * @param extra An object of configuration options for the `FormGroup`.
+   * @param legacyOrOpts Configuration options object for the `FormGroup`. The object can
+   * have two shapes:
+   *
+   * 1) `AbstractControlOptions` object (preferred), which consists of:
+   * * `validators`: A synchronous validator function, or an array of validator functions
+   * * `asyncValidators`: A single async validator or array of async validator functions
+   * * `updateOn`: The event upon which the control should be updated (options: 'change' | 'blur' |
+   * submit')
+   *
+   * 2) Legacy configuration object, which consists of:
    * * `validator`: A synchronous validator function, or an array of validator functions
    * * `asyncValidator`: A single async validator or array of async validator functions
    *