man: describe setting of the clock by systemd and systemd-timesyncd

man: describe setting of the clock by systemd and systemd-timesyncd

The setting of systemd clock is important and deserves an accurate description,
see for example:
https://discussion.fedoraproject.org/t/f38-to-f39-40-dnf-system-upgrade-can-fail-on-raspberry-pi/92403
https://bugzilla.redhat.com/show_bug.cgi?id=2242759

The meat of the description was in systemd-timesyncd.service(8), but
actually it's systemd that sets the clock. In particular, systemd-timesyncd
doesn't know anything about /usr/lib/clock-epoch, and since systemd sets
the clock to the epoch when initializing, systemd-timesyncd would only
get to advance the clock to the epoch under special circumstances.
Also, systemd-timesyncd is an optional component, so we can't even rely
on its man page being installed in all circumstances. The description needs
to be moved to systemd(1).

The description is updated to describe the changes that were made in
previous commits.