Update FilterExpressions.md
Update FilterExpressions.md

Added Alarm Severity Values table.
GridProtectionAlliance/gsf,GridProtectionAlliance/gsf,rmc00/gsf,rmc00/gsf,rmc00/gsf,rmc00/gsf,rmc00/gsf,rmc00/gsf,GridProtectionAlliance/gsf,GridProtectionAlliance/gsf,GridProtectionAlliance/gsf,GridProtectionAlliance/gsf,rmc00/gsf,GridProtectionAlliance/gsf,GridProtectionAlliance/gsf