Clarify whether schemas traversed in path expressions must be in scope

It is sufficient to reference service B from service A. A client/validator needs to read the metadata of service B anyway to find out whether the referenced entity type from service B has a navigation property NavPropToServiceC. Then it can use the references within service B's metadata to figure out where NavPropToServiceC points to, and if necessary follow a reference from service B to service C for the next step of validation. This is true for all path traversal (i.e., in URL) and is in contrast to directly referencing a type defined in service C's schema from service A (which requires a direct reference to service c's schema)