net-p2p/transmission-daemon: Improve UPDATING entry and add pkg-message

This will ensure users who do not read UPDATING are still presented with
the message about how to allow clients to connect to the daemon using
DNS when they upgrade the package.

PR: 225150
Reported by: swills
Security: https://www.vuxml.org/freebsd/3e5b8bd3-0c32-452f-a60e-beab7b762351.html