docs: add membership_lock when creating groups

doc/api/groups.md

@@ -484,6 +484,7 @@ Parameters:
 | `name`                               | string  | yes      | The name of the group. |
 | `path`                               | string  | yes      | The path of the group. |
 | `description`                        | string  | no       | The group's description. |
+| `membership_lock`                    | boolean | no       | **(STARTER)** Prevent adding new members to project membership within this group. |
 | `visibility`                         | string  | no       | The group's visibility. Can be `private`, `internal`, or `public`. |
 | `share_with_group_lock`              | boolean | no       | Prevent sharing a project with another group within this group. |
 | `require_two_factor_authentication`  | boolean | no       | Require all users in this group to setup Two-factor authentication. |