manual: Rewrite the section on widths of integer types.

The manual contradicted itself by saying the number of bits in an
integer type needed to be computed, and then listing a number of
macros that later standards provided for exactly that.  The entire
section has been reworked to provide those macros first, while
preserving the documentation of CHAR_BIT and the associated examples
within that context.

	* manual/lang.texi
	(Computing the Width of an Integer Data Type): Rename section
	to "Width of an Integer Type".  Remove inaccurate statement
	regarding lack of C language facilities for determining width
	of integer types, and reorder content to improve flow and
	context of discussion.