CR — Missing notational element for discretionary Milestones and discretionary EventListeners

Stages and Tasks (generalizations of PlanItemDefinitions) have discretionary notations (dashed lines), but Milestones and EventListeners (also generalizations of PlanItemDefinitions) don't have discretionary notations.
Reading the specification seems like all PlanItemDefinitions can be discretionary, but there is no notation shown for Milestones and EventListeners.
There is no written limitation in the specification that prevents Milestones and EventListeners from being discretionary.
The ability to model discretionary milestones is important in situations where other discretionary items are added.

Add notational element for discretionary Milestones and discretionary EventListeners

Stages and Tasks (generalizations of PlanItemDefinitions) have discretionary notations (dashed lines), but Milestones and EventListeners (also generalizations of PlanItemDefinitions) don't have discretionary notations. Reading the specification seems like all PlanItemDefinitions can be discretionary, but there is no notation shown for Milestones and EventListeners. The ability to model discretionary milestones is important in situations where other discretionary items are added.