Make the language setting string clearer (#531)

lang/en/customcert.php

@@ -34,7 +34,7 @@
 $string['cannotverifyallcertificates'] = 'You do not have the permission to verify all certificates on the site.';
 $string['certificate'] = 'Certificate';
 $string['certificateexecutionperiod'] = 'Certificate execution period';
-$string['certificateexecutionperiod_desc'] = 'Specify the period for which certificates should be executed based on their end date. Set to 0 to execute all certificates, regardless of their age.';
+$string['certificateexecutionperiod_desc'] = 'Specify the period for which certificates should be executed based after the course end date. Set to 0 to execute all certificates, regardless of their age.';
 $string['certificatesperrun'] = 'Certificates per run';
 $string['certificatesperrun_desc'] = 'Enter the number of certificates to process per scheduled task run where 0 means it will process all certificates.';
 $string['code'] = 'Code';