Re-add details about migrating artifacts back to local storage

What does this MR do and why?

The section migrate artifacts to local currently does not contain the command needed to migrate data. Seems it was removed in the commit 02629737, but it's not clear why because this command still exists.

This MR brings the instruction for migrating artifacts to local storage back to the docs.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

doc/administration/job_artifacts.md

@@ -279,8 +279,11 @@ to clean up orphaned artifacts.
 
 ### Migrating from object storage to local storage
 
-To migrate back to local storage, you must
-[selectively disable the artifacts storage](object_storage.md#disable-object-storage-for-specific-features).
+To migrate artifacts back to local storage:
+
+1. Run `gitlab-rake gitlab:artifacts:migrate_to_local`.
+1. [Selectively disable the artifacts' storage](object_storage.md#disable-object-storage-for-specific-features) in `gitlab.rb`.
+1. [Reconfigure GitLab](restart_gitlab.md#reconfigure-a-linux-package-installation).
 
 ## Expiring artifacts