remove or justify all remaining uses of Bytes.unsafe_of_string

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15077 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
document the unsafe conversions Bytes.unsafe_{to,of}_string

Users will inevitably do dirty things during the -safe-string
migration. Better document the available dirty hacks, rather than
letting them use Obj.magic by lack of information.