Clarify an edge case.

proposals/3083-restricted-rooms.md

@@ -48,12 +48,16 @@ Any entries in the list which do not match the expected format are ignored.
 
 When a homeserver receives a `/join` request from a client or a `/make_join` / `/send_join`
 request from a server, the request should only be permitted if the user has a valid
-invite or is in one of the listed spaces. Note that the server may not know if the user
+invite or is in one of the listed spaces.
-is in a particular space, this is left to a future MSC to solve.
 
 If the user is not part of the proper space, the homeserver should return an error
 response with HTTP status code of 403 and an `errcode` of `M_FORBIDDEN`.
 
+It is possible for a homeserver receiving a `/make_join` / `/send_join` request
+to not know if the user is in a particular space (due to not participating in any
+of the necessary spaces). In this case the homeserver should reject the join,
+the requesting server may wish to attempt to join via other homeservers.
+
 Unlike the `invite` join rule, confirmation that the `allow` rules were properly
 checked cannot be enforced over federation by event authorization, so servers in
 the room are trusted not to allow invalid users to join.<sup id="a3">[3](#f3)</sup>