UefiCpuPkg/PiSmmCpuDxeSmm: update comments in IA32 SmmStartup()

UefiCpuPkg/PiSmmCpuDxeSmm: update comments in IA32 SmmStartup()

The gSmmCr3, gSmmCr4, gSmmCr0 and gSmmJmpAddr global variables are used
for patching assembly instructions, thus we can't yet remove the DB
encodings for those instructions. At least we should add the intended
meanings in comments.

This patch only changes comments.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
[lersek@redhat.com: adapt commit msg to ongoing PatchAssembly discussion]