MdePkg DxePcdLib: Updates DxePcdLib instance comments.

Describe PCD service can’t be used at Runtime phase.

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



git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg@16403 6f19259b-4bc3-4df7-8a09-765794883524