development(7): Replace "reboot" with "shutdown -r now"

We generally document shutdown(8) instead of reboot(8) as it's better for
interactive use.

In modern FreeBSD is matters a lot less, it's mostly just convention. One
minor thing is that shutdown(8) produces a global message, while reboot(8)
does not. It is believed that historically, some versions of reboot did not
do appropriate safe shutdown checks and just rebooted.

It's also just consistency: for example the handbook[1] documents shutdown.

There is actually another important difference between reboot and shutdown
-r now: reboot does not run /etc/rc.shutdown. This is because reboot has
its own shutdown procedure and does not signal init like init 6 and
shutdown -r now do (except in the case of rerooting via reboot -r).

A few years ago jilles@ proposed changing reboot's default to signalling
init (preserving reboot -q which just invokes the reboot system call), but
this was not accepted. Perhaps this can be tried again for 13.0.

[1]: https://www.freebsd.org/doc/handbook/boot-shutdown.html

Reported by: eadler
Reviewed by: eadler, jilles
Approved by: krion (mentor)
Differential Revision: https://reviews.freebsd.org/D16843