We’re currently still on Copter 3.5.x and can’t update right now, but the documentation at https://ardupilot.org/copter/index.html is always for the newest version, and I can’t seem to find a link to prior versions of the docs…
Unfortunately that an issue with our documentation, we don’t have release cycle for it and it is always lagging behind software release…
On github interface, you should be able to get the documentation at the date you want and get a zipfile of the whole wiki