manual: Refactor documentation of CHAR_BIT.

manual: Refactor documentation of CHAR_BIT.

This single-@item @table is better defined with @deftypevr, since the
CHAR_BIT macro has @standards (being declared in a header), and @items
in @tables are not considered annotatable.  Using @deftypevr
automatically includes the macro in the Variable and Constant Macro
Index and ensures its inclusion the Summary of Library Facilities.
@deftypevr is used to record the type of the macro so that it also
appears in the Summary.

The description is updated to mention a later POSIX requirement that
this macro have the value 8.

	* manual/lang.texi (CHAR_BIT): Convert from an @table to an
	@deftypevr.  Change standard from ISO to C90.  Mention the
	POSIX.1-2001 requirement of the value 8.