More doc.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13363 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Alternative syntax for some constructions starting with a keyword.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13364 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02