Add a note to release announcements clarifying that all distros may not have the update

I've seen feedback that the release announcements are somewhat confusing to many people since their distros don't have the update, and they think that KDE release announcements should mean a distro update

So, we should add a note to clarify that downstream distros may not have the update yet, and users of those distros will need to wait.

Maybe we can even add a small box showing which distros do have the update (we can use data from repology, etc). That would be a cool thing to have there.