I'm assuming that it's a simple oversight that we don't specify OperationDefinition.resource=DocumentReference. The OperationDefinition does clarify that the operation is at the type level, and not the system or instance level.

In US Core's server CapStmt, we at least imply that the operation exists on DocRef:
>GET [base]/DocumentReference/$docref?patient=[id]

https://build.fhir.org/ig/HL7/US-Core/CapabilityStatement-us-core-server.html#documentreference

Am I missing something? Alternatively, are we intentionally imprecise on which resource $docref is accessed?

Please either clarify that $docref should be called on /DocumentReference, or list the expected resources types on which $docref may be accessed and provide guidance to both server and client developers on where to locate and find this operation.