docs: Update with clarifying link to restui module

Fixes #510

www/content/guides/webform.mdx

@@ -21,7 +21,7 @@ See [https://example-webform.next-drupal.org](https://example-webform.next-drupa
 ## Installation
 
 1. Install the Webform REST module: `composer require drupal/webform_rest`
-2. Visit `/admin/config/services/rest` and enable the **Webform Submit** resource.
+2. Visit `/admin/config/services/rest` and enable the **Webform Submit** resource. You might want to require and enable the [Rest UI](https://drupal.org/project/restui) module for that route to be there.
 3. Select _POST_ for the **Methods**, _json_ for **Accepted request formats** and _cookie_ for **Authentication providers**
 4. **Save configuration**
 5. Next, visit `/admin/people/permissions` and give anonymous users the **Access POST on Webform Submit resource** permission.