clarify instructions for using Switchboard for configuration changes to Dedicated instances

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/165580



Merged-by: Lysanne Pinto <lpinto@gitlab.com>
Approved-by: Lysanne Pinto <lpinto@gitlab.com>
Reviewed-by: Lysanne Pinto <lpinto@gitlab.com>
Co-authored-by: Loryn Bortins <lbortins@gitlab.com>