Add comments for clarification about memset implementation.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Long, Qin <qin.long@intel.com>
Reviewed-by: Ye, Ting <ting.ye@intel.com>
Reviewed-by: Fu, Siyuan <siyuan.fu@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15662 6f19259b-4bc3-4df7-8a09-765794883524