MdePkg PciIo.h: Correct function description for ALLOCATE_BUFFER

MdePkg PciIo.h: Correct function description for ALLOCATE_BUFFER

DUAL_ADDRESS_CYCLE is missing in the EFI_UNSUPPORTED
return status description.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>