It seems that the 'unload-when-stopped' and 'version' subcommands of system-control are not mentioned in the documentation.
They both should be. I've added the latter to the doco ready for the next version. The former I'm wavering about. I've had a to-do item for some time to turn that into "service-control --exit" (a.k.a. "svc -x"), changing it to use the supervise/control API instead of the service manager's control socket. The service manager would do the same thing. Just the API and the control utilities would change. The problems are (a) I haven't got around to that part of the to-do list yet and (b) the upgrade path is a little tricky and I think would involve more than one release.