Clarify that /register must produce valid Matrix User IDs

Fixes https://github.com/matrix-org/matrix-doc/issues/1793

api/client-server/registration.yaml

@@ -59,6 +59,9 @@ paths:
         supplied by the client or generated by the server. The server may
         invalidate any access token previously associated with that device. See
         `Relationship between access tokens and devices`_.
+
+        Any user ID returned by this API must conform to the grammar given in the
+        `Matrix specification <../appendices.html#user-identifiers>`_.
       operationId: register
       parameters:
         - in: query
@@ -142,7 +145,7 @@ paths:
                   The fully-qualified Matrix user ID (MXID) that has been registered.
 
                   Any user ID returned by this API must conform to the grammar given in the
-                  `Matrix specification <https://matrix.org/docs/spec/appendices.html#user-identifiers>`_.
+                  `Matrix specification <../appendices.html#user-identifiers>`_.
               access_token:
                 type: string
                 description: |-

changelogs/client_server/newsfragments/2091.clarification

@@ -0,0 +1 @@
+Clarify that ``/register`` must produce valid Matrix User IDs.