added purge_triggered counter to dyn-stats doc
added purge_triggered counter to dyn-stats doc

qha/rsyslog-doc,sematext/rsyslog-doc,qha/rsyslog-doc,qha/rsyslog-doc,sematext/rsyslog-doc,janmejay/rsyslog-doc,sematext/rsyslog-doc,sematext/rsyslog-doc,janmejay/rsyslog-doc,qha/rsyslog-doc,janmejay/rsyslog-doc,janmejay/rsyslog-doc