[Bug c/80409] Document that va_arg(ap, void*) can be used to consume any pointer argument

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80409

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I don't see why this needs to be documented as it should work even with
standard c. That is for an example %p in printf takes a void* but nobody casts
it to void* when passing to printf.