MdePkg: correct and clarify documentation of VA_LIST in Base.h

MdePkg: correct and clarify documentation of VA_LIST in Base.h

This is to resolve bug 457.
https://bugzilla.tianocore.org/show_bug.cgi?id=457

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zenith432 <zenith432@users.sourceforge.net>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
[lersek@redhat.com: reconstruct commit from patch pasted into email]
Signed-off-by: Laszlo Ersek <lersek@redhat.com>