[RFC] document the unsafe conversions Bytes.unsafe_{to,of}_string

The attached patch contains a tentative documentation of the two new functions [Bytes.unsafe_of_string] and [Bytes.unsafe_to_string] that are intended for use when migrating code to support the new `-safe-string` option.

It is only a preliminary patch; in particular, it does not contain much OCamldoc formatting (I will add those laters). I would be interested in feedback on the content. The objectives of documenting these functions are the following:

1. we do not want beginners to use them (there is no need to, given the existing `Bytes.of_string` and `Bytes.to_string` which are correct and fast enough for most purposes)
2. we do not want experts to use them wrongly or, worse, wrongly use `Obj.magic` or `"%identity"` instead.

Not documenting at all is a very good way to reach goal (1), but does pretty badly on (2). Do you feel the proposed documentation is effective? How should it be improved?

Finally, be aware that documenting unsafe functions is still a controversial topic among OCaml maintainers. I'd like to get as good a proposal as possible, but it is possible that maintainers decide to keep those undocumented in any case. Invest your time wisely.
Well I've seen an expert author use `external foo : ... = "%identity"` in a place where `let foo x = x` would type-check, because it might be faster. I will remove the sentence because "condescending" is not good at all, but I'm worried the documentation may not be very good at making it clear to beginners they need not use those functions. I will try to move the "this is for experts" mention more to the beginning, and maybe reuse some of the worrding in the documentation for [Low-level GC functions](http://caml.inria.fr/pub/docs/manual-ocaml-400/manual033.html#htoc275). Thanks!