there is no document for this since there's no support for this at the moment. If you want to move all users from one box to another, the way to do it is with DiRT by either upgrading the version you're moving from to the version you're moving to or vice versa.
In this case I'd normally I reccomend folks do a DiRT backup of your 3.1(5), restore it onto a new 3.1(5) install which you then upgrade to 4.0(2).
If you were looking to move just particular users between the system (i.e. for load ballancing) then you can't do this. For 4.0(1) and later systems you'll be able to use the Global Subscriber Manager tool to move subscribers around between servers in the same dialing domain but there will be no support for 3.x to 4.x moves.