clarify that Request for Context change re-uses Hub's FHIR identifiers in payload

[isaacvetter]

When a subscribing app requests a context change, it provides an event notification payload, containing one or more FHIR resources. It's unclear where those FHIR resources came from.

The spec should clarify that the FHIR identifiers and perhaps even resources communicated during an event notification and context change request are those ids provided by the Hub's FHIR server.

https://fhircast.hl7.org/specification/Feb2020Ballot/#request-context-change