man: rework systemd-vconsole-setup man page a bit (#7312)

man: rework systemd-vconsole-setup man page a bit (#7312)

It's not systemd that invokes the service internally as needed, it's
systemd-localed. Correct that.

Also, stop using the word "helper". To me a "helper" constitutes
something internal, not official API. I doubt systemd-vconsole-setup
really matches that description though, hence let's better avoid the
term.

Also, clean up some other wording, and be less imbiguous, by suggesting
a single command to apply vconsole.conf instead of two.

Follow-up for 597c25d2a7c61453acd723340350e8748a5106dd