So that you understand FreePBX is a tool for configuring asterisk, not a complete package. That’s what the distros like trixbox, PBX-in-a-flash, Elastix are. So documenting how asterisk functions work like transfer (which can be phone dependent, some have tranfer buttons others don’t), etc are not directly under the FreePBX realm but more directly under those distros and asterisk.
Now that doesn’t mean there is nothing posted; but if and when somebody writes something that can be used it get’s posted for all to see.
Also realize that trixbox has decided to fork FreePBX two months ago and once that happened they started to customize it for their own purposes. Once they did that they become responsible for it, at least until they contribute the changes back so that they can get integrated and documented.
You will probably find some of these items of interest: