Lwt_sequence: point people to lwt-dllist

Part of #361.

src/core/lwt_sequence.mli

@@ -14,13 +14,13 @@
     - removing an element while the sequence is being transversed.
 
    @deprecated This module should be an internal implementation detail of Lwt,
-   and may be removed from the API at some point in the future. Use any other
-   doubly-linked list library as an alternative.
+   and may be removed from the API at some point in the future. Use package
+   {{:https://github.com/mirage/lwt-dllist} [lwt-dllist]} instead.
 *)
 
 [@@@ocaml.deprecated
-" This module is an implementation detail of Lwt. See
-   https://github.com/ocsigen/lwt/issues/361"]
+" Use package lwt-dllist. See
+   https://github.com/mirage/lwt-dllist"]
 
 type 'a t
   (** Type of a sequence holding values of type ['a] *)