Parameters says:

> This resource is a non-persisted resource used to pass information into and back from an operation. It has no other use, and there is no RESTful endpoint associated with it.

... which seems to preclude this resource from being used in the context of a Messaging bundle (as MessageHeader.focus). There doesn't appear to be any other good way to provide named parameters in a Messaging notification, and http://build.fhir.org/messaging even includes examples showing Parameters. So we should remove the prohibitory language from the Parameters intro.

Proposal:

This resource is a non-persisted resource primarily used to pass information into and back from an operation. There is no RESTful endpoint associated with it.