One of the "benefits" of a BPM based solution is that is "should be" self documenting.
If your process has become so unwieldy that the flow and decisions that control that flow cannot easily be understood by simply viewing the process and associated annotations, then you should consider re-architecting your solution as you have lost much of the value associated with BPMN based application.
In our consultancy, solution documentation usually consists of:
Decision Tables and graphs
Unit tests for all integrations and services (test driven design)
Measurements (Key performance indicators)
The majority of this should be able to be taken directly from the modeling environment without any need for building special documentation which needs to be maintained.