Automatic update guide doesn't cover third-party archives

[Henri Sivonen]

https://help.ubuntu.com/12.04/serverguide/automatic-updates.html does not explain how to configure automatic updates for third-party archives. It should. Both for third-party archives that declare Suite and archives that don't.

http://askubuntu.com/questions/87849/how-to-enable-silent-automatic-updates-for-any-repository explains how do deal with archives that declare Origin and Suite but doesn't explain what to do about archives that don't declare Suite.