MdePkg: Enhance the comments for SmmMemoryAllocationLib instance.

MdePkg: Enhance the comments for SmmMemoryAllocationLib instance.

Per PI spec all allocation of SMRAM should use EfiRuntimeServicesCode or EfiRuntimeServicesData.
So SMM instance use EfiRuntimeServicesCode as the default memory allocation type instead of EfiBootServicesData.
This patch clarify this usage in comments.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17666 6f19259b-4bc3-4df7-8a09-765794883524