resolved: tone down comment in /run/systemd/{stub-,}resolve.conf

There are legitimate reasons to access the file directly, as currently
discussed on fedora-devel. Hence tone things down from "must" to "should
typically not".

Also, let's use fputs() instead of fputs_unlocked() here,
fopen_temporary_label() turns off stdio locking anyway for the whole
FILE*, hence no need to do this manually each time.

(cherry picked from commit ce416f42)