The docs should mention that transition relations aren't automatically made available in functions called by the trigger function. This unknown behavior caused a lot of frustration for me.
Seems to fall into the "if it isn't documented it doesn't happen" category of things that we simply choose not to clutter the documentation with. Called functions don't inherit stuff from the caller. It is like documenting a variable in the calling function isn't visible to the called function. Admittedly at least with variables you can pass them via arguments. The special trigger state is indeed available in the trigger returning function when it is the function directly invoked by the trigger.