add small precision about -warn-error

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14760 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
fix various things

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14761 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02