I'm an owner on all documents that I create, and whenever I share with someone, I usually give them at most "editor" permissions. So, I'd like to find all documents in which I have given another user "owner" permissions in the past, and review them to see if I want to downgrade them to "editor".
Here's an answer, though you might not like it:
Once you transfer ownership of a file, you lose ownership for yourself. You effectively change the doc from something you own to something that was "shared with you."
There is no way to downgrade a user back to editor unless you contact them to make you the owner again (you can't complete this change once you transfer ownership)