Hi Marton,
The list you are searching it in the following address, for jBPM5.2:
http://localhost:8080/gwt-console-server/rs/server/resources/jbpm
It is just a list of REST services that can execute. Although, I must say it is not 100% accurate. (but if I'm wrong someone can correct me!)
For example,
The delete operation is stated as, /gwt-console-server/rs/engine/deployment/{id}/delete
but in reality is /gwt-console-server/rs/process/instance/{id}/delete
I don't know about official documentation, but what I'm using is debuging through the gwt-console! It uses the REST interface and at the bottom has a messages console that will tell you all the calls it is doing!
But I have learned that you have a couple of jira issues to address this:
https://issues.jboss.org/browse/JBPM-3577
https://issues.jboss.org/browse/JBPM-3212