Specification referencing self-clarification outside the specification itself?

Within the “Change Management and Versioning” page, in section Maturity Levels, a statement reads:
“The Maturity level is strongly related to stability; the higher the maturity level, the more controls are enforced to restrict breaking changes to the resource. For further information, and discussion, see the FHIR Confluence page .”

Referencing external content that supports a concept or paradigm within the specification would seem acceptable, but should the FHIR specification reference FHIR-specific clarification materials that lie outside of the specification itself (in this example a Confluence FHIR page)?

If the material is worthy of being referenced for the understanding of the standard, shouldn’t that be within the spec itself?

The FHIR specification defines what’s need to implement FHIR. It does not (and should not) define the methodology used to design FHIR. All of the key elements an implementer needs to understand FHIR maturity metrics and their relevance to implementation is included in the FHIR specification. Are there elements in the Confluence page that are missing from the core spec you feel are essential to implementation?