Improve SSH database key lookup documentation
In gitlab-shell#118 (closed) we added a new authorized_keys lookup script. Now we need to update the documentation in https://docs.gitlab.com/ee/administration/operations/fast_ssh_key_lookup.html to refer to it.
Database key lookup is currently EE-only but is being ported to CE in %10.4: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16014 . We should make the change in CE, after the port is complete.