PR#6717: Pprintast does not print let-pattern attributes

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15677 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
notes on attributes in pprintast.ml

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15678 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02