`changelog-d` was changed to make `packages` a list, so reflect
this in the example. Also remove the "omit if it's an overarching
change" part: we must list all packages if we want it to show up
when generating release changelogs.

While I was there, I updated the Matrix channel reference because
it was still reflecting the EMS Libera bridge.

Co-authored-by: Artem Pelenitsyn <a.pelenitsyn@gmail.com>