Add a note about resident servers.

proposals/3083-restricted-rooms.md

@@ -98,6 +98,12 @@ caveat that servers must ensure that:
       content with the key `join_authorised_via_users_server`. The actual user
       chosen is arbitrary.
 
+      This creates a new restriction on the relationship between the resident
+      servers used for `/make_join` and `/send_join` -- they must now both go to
+      the same server (since the `join_authorised_via_users_server` is added in
+      the call to `/make_join`, while the final signature is added during
+      the call to `/send_join`).
+
 Note that the homeservers whose users can issue invites are trusted to confirm
 that the `allow` rules were properly checked (since this cannot easily be
 enforced over federation by event authorisation).<sup id="a4">[4](#f4)</sup>