MdePkg: Correct comments of macros FixedPcdGetX/PatchPcdXXX in PcdLib.h
MdePkg: Correct comments of macros FixedPcdGetX/PatchPcdXXX in PcdLib.h

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=311

The patch is also to refine comments of macros PcdToken and PcdTokenEx.

Cc: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>
Cc: Michael Kinney <fd20bc543a9f65bb633fac7d08a403c1f1c8eb6c@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <5c6bb72efe464613af94e84c05522a361b23844a@intel.com>
Reviewed-by: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2