Getting to the documentation for upgrading Foreman with Katello is a bit difficult.
If you start at The Foreman Manual: Foreman :: Manual you can find this link that says if you are using Katello go here: Foreman :: Plugin documentation index. That takes you to a page that at the top says: Katello’s documentation for versions 4.0+ has moved to docs.theforeman.org. And if you follow that you never get to the documentation for Katello.
I do not even know how I got there but some how I managed to get to this page Upgrading Foreman to 3.14. And from there I can choose the version of Foreman/Katello I am using and get the correct information.