Can't help with the documentation, but you can start with this query:
select obj.* from t_object obj, t_diagramobjects dob, t_diagram dia
where obj.object_id = dob.object_id
and dob.diagram_id = dia.diagram_id
and dia.Diagram_ID = (SELECT pdata1 FROM t_object where object_type="UMLDiagram")
This lists all elements of all diagram frames. The last part selects the diagrams referenced by diagram frames.
q.