MdeModulePkg PeiCore: Improve comment semantics

MdeModulePkg PeiCore: Improve comment semantics

This patch clarifies wording in several PeiCore comments to improve
reading comprehension.

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>