manual: Update @standards for [__]va_copy.

The ISO version in which va_copy was introduced is made explicit, and
__va_copy is given @standards.  The description is updated to be more
clear about the origins of each macro, and the reader is informed
these macros are now provided by the compiler (information previously
embedded in a Texinfo @comment).

	* lang.texi (va_copy): Change standard from ISO to C99.
	(__va_copy): Add standard and header annotation.
	Update description for clarity of origins and current use.