[PATCH][BZ #6885] Clarify description of __freading and __fwriting

[Roland McGrath]

> +The @code{__freading} and @code{__fwriting} functions exist primarily for use
> +by gnulib and for other legacy software.  Their use in new programs is not
> +recommended.

If we were to mention gnulib it would belong in @file or something.  But I
don't think we should use wording like this.  If we want to discourage use
of an interface, the the manual should describe why it's suspect to use it
and what patterns we recommend instead that make its use unnecessary.