allow adding canonical aliases from remote servers

Like is mentioned in the comment, this isn't explicitly required by the
spec, but it's reasonable and what synapse does.

src/api/client_server/alias.rs

@@ -107,6 +107,8 @@ pub(crate) async fn get_alias_route(
     get_alias_helper(body.body.room_alias).await.map(Ra)
 }
 
+// Can't use `services().rooms.alias.resolve_alias` because we also need the set
+// of servers from the remote get_room_information request.
 pub(crate) async fn get_alias_helper(
     room_alias: OwnedRoomAliasId,
 ) -> Result<get_alias::v3::Response> {