manual: Refactor documentation of CHAR_BIT.

[Zack Weinberg]

On 06/20/2017 08:11 AM, Florian Weimer wrote:
> 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.

Specifically, that requirement was added in POSIX.1-2001 (aka Issue 6), per
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html.

I agree that this should be mentioned.

zw