... and this documentation has just been extended significantly ;)
The documentation contains a hello world example using BPMN2:
https://hudson.jboss.org/hudson/job/jBPM5/lastSuccessfulBuild/artifact/target/jbpm-5.0-SNAPSHOT-docs-build/jbpm-docs/html/ch03.html
If you use the installer, you can also see a simple human task process in action in the various tools.