Document npm package requests forwarding

doc/user/admin_area/settings/continuous_integration.md

@@ -191,3 +191,16 @@ To set required pipeline configuration:
 1. Click **Save changes**.
 
 ![Required pipeline](img/admin_required_pipeline.png)
+
+## Package Registry configuration **(PREMIUM ONLY)**
+
+GitLab administrators can disable the forwarding of NPM requests to [npmjs.com](https://www.npmjs.com/).
+
+To disable it:
+
+1. Go to **Admin Area > Settings > CI/CD**.
+1. Expand the **Package Registry** section.
+1. Uncheck **Enable forwarding of NPM package requests to npmjs.org**.
+1. Click **Save changes**.
+
+![NPM package requests forwarding](img/admin_package_registry_npm_package_requests_forward.png)

doc/user/packages/npm_registry/index.md

@@ -269,6 +269,14 @@ Or if you're using Yarn:
 yarn add @my-project-scope/my-package
 ```
 
+### Forwarding requests to npmjs.org
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/55344) in [GitLab Premium](https://about.gitlab.com/pricing/) 12.9.
+
+By default, when an NPM package is not found in the GitLab NPM Registry, the request will be forwarded to [npmjs.com](https://www.npmjs.com/).
+
+Administrators can disable this behavior in the [Continuous Integration settings](../../admin_area/settings/continuous_integration.md).
+
 ## Removing a package
 
 In the packages view of your project page, you can delete packages by clicking