Fix the description of filter switch

tools/bufr_filter.sh

@@ -136,7 +136,7 @@ echo "}"
 echo "\\endverbatim\\n"
 echo "Each value of each key given as argument to the switch statement is matched against the values specified in the case statements.\\n"
 echo "If there is a match, then the block or rules corresponding to the matching case statement is executed.\\n"
-echo "Otherwise, the default case is executed. The default case is mandatory, even if empty.\\n"
+echo "Otherwise, the default case is executed. The default case is mandatory if the case statements do not cover all the possibilities.\\n"
 echo "The \"~\" operator can be used to match \"anything\".\\n\\n"
 
 

tools/grib_filter.sh

@@ -116,7 +116,7 @@ echo "}"
 echo "\\endverbatim\\n"
 echo "Each value of each key given as argument to the switch statement is matched against the values specified in the case statements.\\n"
 echo "If there is a match, then the block or rules corresponding to the matching case statement is executed.\\n"
-echo "Otherwise, the default case is executed. The default case is mandatory, even if empty.\\n"
+echo "Otherwise, the default case is executed. The default case is mandatory if the case statements do not cover all the possibilities.\\n"
 echo "The \"~\" operator can be used to match \"anything\".\\n\\n"
 echo "Following is an example showing the use of the switch statement:\\n"
 echo "\\verbatim"