manual: Refactor documentation of CHAR_BIT.

[Zack Weinberg]

On 06/17/2017 04:42 AM, Rical Jasan wrote:
> This single-@item @table is better defined with @defvr, since the
> CHAR_BIT macro has @standards (being declared in a header), and @items
> in @tables are not considered annotatable.  Using @defvr automatically
> includes the macro in the Variable and Constant Macro Index and
> ensures its inclusion the Summary of Library Facilities.

This part of the change is OK.

> The file include/limits.h identifies the macro as coming from C99.

I think this macro really was in C90, and
http://flash-gordon.me.uk/ansi.c.txt includes it, which is the closest I
can come to checking.  Joseph, do you know for sure?

Note that the comment at the top of the header file

/*
 *      ISO C99 Standard: 7.10/5.2.4.2.1 Sizes of integer types <limits.h>
 */

should _not_ be taken to imply anything about which standard specifies
which bits of the header.  It's meant to point you to the core _overall_
specification for the header, and we seem to consistently refer to C99
for that even when the header existed in C90 (compare libio/stdio.h, for
instance).

zw