Spec reference relationships (#1206)

* Spec reference relationships

MSC: https://github.com/matrix-org/matrix-spec-proposals/pull/3267

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Edits per code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

data/event-schemas/schema/m.key.verification.m.relates_to.yaml

@@ -10,7 +10,8 @@ properties:
     enum:
       - m.reference
     description: |-
-      The relationship type.
+      The relationship type. Currently, this can only be an
+      [`m.reference`](/client-server-api/#reference-relations) relationship type.
   event_id:
     type: string
     description: |-