MdeModulePkg/MdeModulePkg.dec/.uni: clarify PCDs usage

MdeModulePkg/MdeModulePkg.dec/.uni: clarify PCDs usage

BZ#1116: https://bugzilla.tianocore.org/show_bug.cgi?id=1116

The usage of following PCDs described in MdeModulePkg.dec don't match
the implementation exactly. This patch updates related description in
both .dec and .uni files to avoid confusion in platform configuration.

  PcdSetNxForStack
  PcdImageProtectionPolicy
  PcdDxeNxMemoryProtectionPolicy

The main change is at the statement on how to handle the FALSE or 0
setting value in those PCDs. Current statement says the implementation
should unset or disable related features but in fact the related code
just do nothing (leave it to AS-IS). That means the result might be
disabled, or might be not. It depends on other features or platform
policy.

For example, if one don't want to enforce NX onto stack memory, he/she
needs to set PcdSetNxForStack to FALSE as well as to clear BIT4 of
PcdDxeNxMemoryProtectionPolicy.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.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>