man: stop recommending putting myhostname after dns

nss-resolve also looks in /etc/hosts, and has the same local hostname
resolving logic as nss-myhostname. We shouldn't recommend another order
than nss-resolve uses internally.

When nss-resolve is used, there's no possibility to override
nss-myhostname hosts via DNS *anyway*.

On top of that, it's not a good idea to allow DNS to override local
hostnames as all - at least not something we should advertise in the
docs.

Followup of f918c67d38ba6ccd4eb0dc657f3f3155e5010cae /
https://github.com/systemd/systemd/pull/16754.