I think the documentation is quite easy to misunderstand here.. But anyway - thanks for the clarification!
And I get it now, I see that if I throw an ActionProcessingFaultException with a message as parameter, and edit the Fault on this message, then my own Fault is returned to the client instead of the default urn:action/error/actionprocessingerror
We'll update the programmer's guide in 4.2.1 to make it clearer.