Clarify git housekeeping

This changed in 6bcc52a5 - The current docs are not clear in stating that we also run a git repack and that we respect the admin application setting values.

See:

• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/services/projects/housekeeping_service.rb#L68-76
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/workers/git_garbage_collect_worker.rb#L29-42