firstboot: clarify that machine-id options are only offline, add missing docs

firstboot: clarify that machine-id options are only offline, add missing docs

Let's flat out refuse to configure machine-id on a running system with
systemd-firstboot. It wouldn't work anyway, because by the time firstboot is
started, pid1 has created /etc/machine-id, possibly with "unitialized", so
firstboot wouldn't touch the file. (If --force is specified, it works. So
let's allow that in case people want to do crazy things.)

While at it, add missing descriptions of various things that were added over
time, and group descriptions of similar options together.