MdeModulePkg: cleanup Heap Guard pool/page type PCD documentation

This cleanup is meant for avoiding misuse of newly introduced BIT4
(UAF detection) of PCD PcdHeapGuardPropertyMask, because it applies
to all types of physical memory. In another words,
PcdHeapGuardPoolType and PcdHeapGuardPageType don't have effect to
the BIT4 of PcdHeapGuardPropertyMask.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>