On all of them might be best I think. That way, if someone googles, and finds the old docs, no matter which one they go to, they get a link to the latest. Ideally, a link to the latest version of the same function/class/etc. If that class/function no longer exists, maybe a link saying "this is deprecated", and just points to the top page of the latest docs.
Sounds like a lot of not very fun work, but, hey, that's what the website is on github for, for pull requests, right? Haha.