can you explain, why you need documentation for this services?
As it is an internal one (as already pointed out), we are not supposed to use them directly.
You can open the service in Designer to introspect its input and output signature and to search for any other services which are using it.
If I remember right this one is used internally by PRT when one step finishes and the transition is pblished to Messaging to inform the Runtime to start the next step for this instance (at least this is what the name suggests). So there is a Transition Trigger in the Model runtime implementation package, which invokes this service when it receives a document for this model id.
Eventually the BPM Development Guide contains more inforamtions on how this intended to work.