CMMN — Sentries SHOULD be allowed for DiscretionaryItem's (CMMN-8)

It has turned out that many of our potential users of CMMN 1.0 naturally model
discretionary items with Sentries associated to PlanItem's in the Stage and there are multiple
use cases that would benefit from allowing Sentries on Discretionary Items.

So we'd propose to allow Sentries on DiscretionaryItem's in a Planning Table with the restriction
that those Sentries MUST refer PlanItem elements of the Planning Table Stage.

Agreed on the issue, modify PlanningTable metamodel (Figure 10) in section 5.4.9 and associated text in section 5.4.9.2 (DiscretionaryItem class)