man: explain route-only domains a bit more

man: explain route-only domains a bit more

The details discussion of how search and route-only domains work is in
systemd-resolved.service(8). But users are more likely to look at
resolved.conf(5), because that's where Domains= is described. So let's add a
reference to the other man page there, and also strengthen the text a bit. In
particular, in systemd-resolved.service(8) we say "route-only", which makes
the distinction with search domains clearer. Let's use the same in the other
man page too.

This is based on feedback from Lukáš Nykrýn that the man page is not clear
enough.