Add comment for S3BootScriptSaveMemPoll API, since it does not match PI specification.

Signed-off by: Jiewen Yao <Jiewen.yao@intel.com>
Reviewed by: Star Zeng <Star.Zeng@intel.com>

Contributed-under: TianoCore Contribution Agreement 1.0

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@15280 6f19259b-4bc3-4df7-8a09-765794883524