Add comments to describe fields of MSEG_HEADER and add define values for the MonitorFeatures field. Cc: Jiewen Yao <jiewen.yao@intel.com> Cc: Jeff Fan <jeff.fan@intel.com> Cc: Feng Tian <feng.tian@intel.com> Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Michael Kinney <michael.d.kinney@intel.com> Reviewed-by: Jiewen Yao <jiewen.yao@intel.com> Reviewed-by: Jeff Fan <jeff.fan@intel.com>