doc: systemd should only kill master in example
By default, systemd kills every process in the control group
when stopping a service. While it ought to be harmless to
signal workers, some Rack applications (and perhaps further
subprocesses) can misbehave when interrupted by a signal.
Ensure we only hit the master on graceful shutdown to avoid
tickling bugs in Rack apps.
This is the reason we switched to having the master send
"fake" signals for workers beginning with unicorn 4.8.0
back in 2013/2014.