Explicit representation of guards, get rid of Pexp_when.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13528 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Document when Ptyp_poly can appear.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13529 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02