man: deemphasize "halt"

man: deemphasize "halt"

Systemd documents "halt" as the primary shutdown mechanism, redirecting
"reboot" and "shutdown" to the halt(8), but halt is a really strange and
obsolete concept. Who would want to really keep their machine running after
shutdown? I expect that halting is almost unused. Let's at least make it less
prominent in the docs.

While at it, use "power off" for a verb and "power-off" for noun (but "poweroff"
of the actual command name).