PR#6267: more information printed by "bt" command of ocamldebug

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14416 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#6183: enhanced documentation for 'Unix.shutdown_connection'

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14419 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02