* man: Advertise systemd-time-wait-sync.service more
The description of time-sync.target says that NTP services *should* pull
that target, but doesn't mention that e.g. systemd-timesyncd.service
doesn't actually do that. As a result, time-sync.target is reached way
earlier than people expect; see #5097, #8861, #11008.
systemd provides systemd-time-wait-sync.service to ameliorate this
problem, but doesn't feature it prominently in relevant manpages. In
fact, it's only mentioned in passing in systemd-timesyncd.service(8). As
a result, I ended up re-implementing that service, and I'm not the first
one: https://github.com/NixOS/nixpkgs/pull/51338
This patch adds a mention right in the description of time-sync.target,
which will hopefully raise awareness of this helper service.
