Bpmn-moddle: How to add a bpmndi:BPMNShape to a bpmndi:BPMNPlane

How can I add a bpmndi:BPMNShape to a bpmndi:BPMNPlane in bpmn-moddle? Going by the diagram descriptors documenation bpmndi:BPMNPlane doesn’t have a property shape or something similar. Or should I add it in another way?

PS: I’ve asked this question in the comments of my previous question, but since it’s a separate question I’m posting it here as well.

The only example I could find if from here. Apparently there is a planeElement property containing a list of shapes. That works, but I would like to know where I can find the documenation for that.