Clarify what requests can be delta enabled

Clarify that any GET request to retrieve one or more entities MAY be delta enabled by the service. A service MAY support delta enabling additional GET requests. Add Singleton, Navigation Property, and Funtion to the AppliesTo of the ChangingTracking term. Reasoning: The entity set scenario is already well documented so nothing new there. The reason for the single entity case is mainly for singletons. If a service has a singleton, a natural defining query would be for the singleton entity possibly with expands. You could make the same argument for just single entities that are not singletons. The main reason I see for allowing a single entity (singleton or otherwise) without any nested entities to be delta enabled is in the case where the entity is very large (i.e. lots of large properties). In that case, the delta protocol could be used to return only the properties that have changed. A function can return a collection of entities so it should be able to return a delta link. Add Singleton, NavigationProperty, and Function to AppliesTo of ChangeTracking term.