Documentation about supporting update channels

I saw two brief hints to *update channels* in the documentation about [AppImage updates](https://docs.appimage.org/packaging-guide/optional/updates.html?highlight=channel#recommended-user-experience)

> Releases should always update to releases, nightlies always to nightlies, etc. (“channels”)

> Whenever` the application encounters issues (e.g., a crash reporter comes up) it could ask the user to check for updates, and accept bug reports only if no newer version is available in the channel

But it didn't find any more information on *update channels*. I'm currently trying to add an update system supporting channels on an AppImage I'm working on and such information might be usefull. I'm willing to propose a PR with such documentation if I can gather info about it.

As far as my understanding goes, there is no builtin channel support in the AppImage spec or any update tool. The maintainers of an app should upload multiple AppImages on their servers and can choose to "target" the update link on whichever file corresponds to the latest AppImage of the selected channel ? Or is there another way to support *update channels* I'm not aware of ?

If anyone is aware of a project that already has such a system in place I'm interested in looking into it and to document at least one good use case of such a workflow.