doc: correctly document how to terminate old builders
    
    The old method with --kill-also-unused is not working, we changed the
    VM starting layout so everything goes through resalloc server.
    
    For garbage-collector we use a different script for some time:
    https://pagure.io/fedora-infra/ansible/blob/main/f/roles/copr/backend/files/cleanup-vms-aws-resalloc