Add configuration based WorkItemHandler registration to section 14.2.4 of docs

Section 14.2.4 Executing service nodes should include information about registering WorkItemHandlers via configuration. Namely, that you can use a drools.session file to point to a conf file

drools.workItemHandlers = workItemHandlers.conf 

Then inside workIemHandlers.conf define the class file for your user service:

[
	
  "MyUserItem" : new com.sample.SampleWorkItemHandler()
  	
]

Currently, it only lists a programmatic way of WorkItemHandler configuration, which does not work if you want to use your UserService withing a workflow from the jBPM-console.