Document systemd interactions with SDP init scripts.
On operating systems that use systemd as the init mechanism,
admins must be trained to only use systemd to start and stop p4d,
and SDP documentation must clarify this.
This is a somewhat dangerous situation, where systemd doesn't know
p4d is up, but it is. The danger is not immediate. It wont impact p4d
functionality or performance when running. This danger is that, if the
machine were rebooted, p4d would not be asked by the OS to shutdown
gracefully, and that can cause data corruption if the server is busy
at the time of reboot.
This documentation may be unnecessary/obsolete if SDP-428 is