PR#5851: refine 4.02@15111 to print the hint at most once

(Report by Jonathan Protzenko, ack'ed by Damien Doligez)

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15127 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Expose Obj.extension_slot (Jérémie Dimino, ack'ed by Damien)

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15128 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
improve Changelog

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15129 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02