I've just updated the documentation of jBPM5, so I hope that already helps a lot.

I would recommend using jBPM5, as that is the version that development will focus on in the future.  If you're starting out now, it probably makes more sense to look at jBPM5 than jBPM4 or Drools Flow to avoid a migration again in the near future.  jBPM5 is mostly built from existing components and not written from scratch so the current version should already match most of the functionality that could be found in both jBPM4 and Drools Flow, and should be pretty stable already.