rec: document file extension of zone forward file and do not accept empty list of...

rec: document file extension of zone forward file and do not accept empty list of forwards

The YAML code already disallows an empty forwarders list. The added
case mainly covers the case when an old-style forward file is parsed
as if it's YAML. This now generated an error:

Feb 21 09:57:39 msg="Fatal error" error="empty list of forwarders for domain '- zone: example.com\"on line 1 of tmp/f.conf" subsystem="config" level="0" prio="Critical" tid="0" ts="1740128259.137" exception="PDNSException"

This hopefully makes admins realize rec is trying to parse YAML
content, but is expecting old-style as the file does not end in
in .yml.