[PATCH v2] LangRef: clarify that va_arg doesn't work on x86_64

[Ramkumar_Ramachandr1]

Since x86_64 is easily the most popular platform, it deserves special
mention. varargs on x86_64 probably requires a separate page.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>