manual: Refactor documentation of CHAR_BIT.

[Florian Weimer]

On 06/17/2017 10:42 AM, Rical Jasan wrote:
> +@defvr Macro CHAR_BIT
> +@standards{C99, limits.h}
>  This is the number of bits in a @code{char}---eight, on most systems.
>  The value has type @code{int}.

Current POSIX requires CHAR_BIT to be 8, and we might as well say so.

Thanks,
Florian