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.

(Documentation written with helpful feedback of Daniel Bünzli,
Anil Madhavapeddy, and Damien Doligez.)

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15108 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02