Clarify error conditions.
@@ -66,8 +66,8 @@ an error response with HTTP status code of 403 and an `errcode` of `M_FORBIDDEN`
It is possible for a resident homeserver (one which receives a `/make_join` /
`/send_join` request) to not know if the user is in some of the allowed rooms (due
to not participating in them). If the user is not in any of the allowed rooms that
-are known to the homeserver it should return an error response with HTTP status code
-of 400 with an `errcode` of `M_UNABLE_TO_AUTHORISE_JOIN`. The joining server should
+are known to the homeserver, and the homeserver is not participating in all listed
+rooms, then it should return an error response with HTTP status code of 400 with an `errcode` of `M_UNABLE_TO_AUTHORISE_JOIN`. The joining server should
attempt to join via another resident homeserver. If the resident homeserver knows
that the user is not in *any* of the allowed rooms it should return an error response
with HTTP status code of 403 and an `errcode` of `M_FORBIDDEN`. Note that it is a
