Clarify wording for OnDelete

The OnDelete element is intended to specify what happens to entities related by the containing navigation property when the entity containing the navigation property is deleted.

In the current specification we refer to the entity containing the navigation property as the "source" entity, but that term is not well defined as we have used that term in the past to indicate directionality of a relationship, not necessarily the object on which something is defined. Maybe say something like "A navigation property MAY define one edm:OnDelete element. It describes the action the service will take on related entities when the entity on which the navigation property is defined is deleted."

We could be even more clear that it is the entities related according to the navigation property (not all related entities), but hopefully that should be apparent?