I would like to display the documentation in the diagram, or at least be able to display the documentation when I export the diagram to an image file. Is there an option, extension or approach to achieve that?
BPMN doesn't have a graphical representation of the "documentation" property of a diagram, so nothing out of the box here. You can try adding a text-annotation or extending the ProcessDiagramGenerator to render the "documentation" as part of the created .png-file.
