man: move 'files' module in NSS 'hosts:' line before myhostname

I am pretty sure /etc/hosts (i.e. an explicitly configured, local,
trusted database) should be useful for overriding the automatic
myhostname logic.

resolved's internal logic handles it that way and hence we should
suggest it in the NSS fallback line, too.

Let's also bring the factory file back into sync with what the docs say.

And update the prose a bit too, to actually match what we recommend.