Document how Infrastructure/Admins can manage and check the state of Background migrations in the Gitlab.com deployment

Follow up from gitlab-org/charts/gitlab!3928 (comment 2147234335)

The background migration CLI is part of the registry binary and will likely be administered manually from the registry pod (as we do for registry imports today). Some documentation is definitely needed to communicate this.

Also evaluate documenting the queries to run against the database without the binary