Merge branch 'andyschoenen/update-batched-backgrouond-migration-docs' into 'master'

Add milestone example for finalizing migrations

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175869



Merged-by: Leonardo da Rosa <ldarosa@gitlab.com>
Approved-by: Bojan Marjanovic <bmarjanovic@gitlab.com>
Approved-by: Leonardo da Rosa <ldarosa@gitlab.com>
Reviewed-by: Leonardo da Rosa <ldarosa@gitlab.com>
Co-authored-by: Andy Schoenen <aschoenen@gitlab.com>