doc: Fix up return type of __builtin_va_arg_pack_len [PR108560]

__builtin_va_arg_pack_len as implemented returned int since its introduction
in 2007.  The initial documentation didn't mention any return type,
which changed in 2010 in r0-103077-gab940b73bfabe2cec4 during some
documentation formatting cleanups
https://gcc.gnu.org/legacy-ml/gcc-patches/2010-09/msg01632.html
I can understand that for formatting some type was needed there
but what exactly hasn't been really discussed.

So, I think we should change documentation to match the implementation,
rather than change implementation to match the documentation.
Most people don't use more than 2147483647 arguments to inline functions,
and on poor targets with 16-bit ints I bet even having more than 65535
arguments to inline functions would be highly unexpected.

2023-01-27  Jakub Jelinek  <jakub@redhat.com>

	PR other/108560
	* doc/extend.texi: Fix up return type of __builtin_va_arg_pack_len
	from size_t to int.