MdePkg PcdLib: Except for SizeOfBuffer is greater than the maximum size supported, align the behavior of LibPcdSetPtr with LibPcdSetN to handle error status returned.

Let the comments to be aligned with code behavior for LibPcdSetX APIs.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed by: Liming Gao <liming.gao@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg@15056 6f19259b-4bc3-4df7-8a09-765794883524