UML25 — Labels for generalization sets in UML DI

Annex B should explain how to model labels of generalization sets.

Add that generalization set constraint text are modeled as UMLLabels with generalization sets as modelElements,
and that these labels and the ones for power types are owned by their generalization or generalization
set edges. Also add that generalization set lines are modeled as UMLShapes with generalization sets as
modelElements.