Make it clear how to group filters and criteria with the resources they apply to

[daliboz]

Related jira for R5: https://jira.hl7.org/browse/FHIR-28490
Related conversation from connectathon: https://chat.fhir.org/#narrow/stream/179229-subscriptions/topic/Modeling.20resource.20triggers

Thoughts for R4 Backport approach:
Need to decide on syntax
(1) keep multiple values in critieria with an additional separator: "Observation?patient=123 | Encounter?patient=123"
Note that common separators are not available since they are allowed in the values (e.g., patient=123,456 or category=system|code)
(2) leave the first criteria in 'Subscription.criteria' and put additional ones in an extension on that field
(3) assign static text to 'Subscription.criteria' and put all filters in an extension