Add clarification for PcdAcpiDefault value PCD.

Add clarification for PcdAcpiDefault value PCD.

Add clarification for PcdAcpiDefaultOemTableId, PcdAcpiDefaultOemRevision,
PcdAcpiDefaultCreatorId, PcdAcpiDefaultCreatorRevision.
According to ACPI spec, they are used as special meaning for SSDT,
so we ignore them in updating generic SSDT provided by EDKII.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: "Yao, Jiewen" <jiewen.yao@intel.com>
Reviewed-by: "Zeng, Star" <star.zeng@intel.com>com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19321 6f19259b-4bc3-4df7-8a09-765794883524