Clarify mandatory config files in /etc

Add the /etc/cron.hourly directory to the list of cron directories.
/etc/cron.* files now must be configuration files (upgraded from
should).  Mention the various scripts that must be configuration files
in the section about configuration files.  Thanks, Frank Küster.
Closes #403391.

git-archimport-id: rra@debian.org--lenny/debian-policy--devel--3.7--patch-42