UefiCpuPkg: Explain relationship between several SMM PCDs

UefiCpuPkg: Explain relationship between several SMM PCDs

There are three PCDs that may impact the behavior of each other in
SMM environment:
  PcdCpuSmmProfileEnable
  PcdHeapGuardPropertyMask in MdeModulePkg
  PcdCpuSmmRestrictedMemoryAccess

The patch updates the comments in DEC file to document it.

Signed-off-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>