Update "Use of the reference datatype"

The FHIR IG has a section on the "Use of the reference datatype". There are several issues with this section which might need some clarification:

• "Each reference may be either internal or external." It is not explained what this means. From the FHIR documentation, it seems like an internal reference means a reference to a contained resource. Is it useful to mention this disticintion here?

• "The datatype supports a display attribute, comparable to a link title in a web page, and an identifier." This misses the reference attribute, which is the most important one, at least in the MedMij context.

• "The identifier acts as a logical reference when the literal reference is not known. Literal references work based on the FHIR logical id, not the identifier, attribute." True, but very confusing as "resource.id" is the "logical identifier", which is not what identifier referst to. So it's betst to avoid the term "logical". Plus a link to the section on id's can be added.

• In the table, the flags column provides very little added information.

• "The expectation within MedMij is therefore that the vast majority of references will be based on internal references or identifier." This is not true.

• "The reference, if used, SHALL be resolvable". Clarify that this is the reference attribute

• Then there are two notes, one "in-line" and the other referred to from the table. This is a bit messy.

• There is a bullet "Literal (internal/external) references are preferred over logical references when a choice in resource type is available". It is unclear where this belongs to.