What about jBPM5 user manual?: http://docs.jboss.org/jbpm/v5.3/userguide/ch.core-basics.html#d0e1926
Maybe is not an exptensive documentation but at least you can use it to start playing with this type of node.
Do you have any particular problem with this type of sub-process?
I had read the JBPM5 user manual, but it is more useful to create a sub-process using eclipse process editor. In the new eclipse diagram editor things are changed and the document is no longer useful in creating sub-process using diagram editor.