Better representation of with-constraints.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13540 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Doc.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13541 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02