doc: systemd examples should only kill one process

doc: systemd examples should only kill one process

For our daemons, killing only the master process is enough.
Killing the entire control group (as done by default in
systemd) may cause subprocesses such as git to shut down
unexpectedly.

Having systemd kill workers directly will also cause an
immediate shutdown since the master would've already signaled
the workers; and workers will die after two shutdown requests.