Fix PR#5802

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@13927 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#3236: Document the fact that queues are not thread-safe

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@13929 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02