Upsert: clarify upsert along nullable single-valued navigation path

PUT or PATCH to a single-valued navigation property reference (i.e., customers/salesperson/$ref) MUST only include the entity id, and changes which entity is referenced by the navigation property. PUT or PATCH to a single-valued navigation property (i.e., customers/salesperson) performs an update on the currently related resource. If the value of the navigation property is non-null then any key values are ignored and the referenced entity is updated. If the value of the navigation property is null, for a containment relationship a resource is created (since the single-valued navigation path identifies the resource). For a non-containment relationship, PUT or PATCH to a null single-valued navigation property returns an error. The same semantics apply to singletons on the entity container, which are semantically contained single value navigation properties.