Hi,
I'm still trying to figure out some stuff about jbpm5, but here's what I've got so far concerning your question.
In order to insert the content data in a task, start a process (which has a variable declared "somevalue") passing the map of parameters populated with it (let's say params.put("somevalue", somevalue)), and the human task field "Content" (using the Properties view of eclipse) configured to be "#{somevalue}".
This 'somevalue' is (it seems) what is retrieved when you call client.getContent(taskData.getDocumentContentId(), handler).