NMS-6689: Documentation StrafePingMonitor

to the best of my knowledge
@indigo423 
It is written that the `rrd-repository` property uses per default `/opt/opennms/share/rrd/response` and my only complain is, if that is correct, or if the default is `$OPENNMS_HOME/share/rrd/response` to cover the fact that the $OPENNMS_HOME depends on the OS/Distribution.

That is why I asked `shouldn't it be $OPENNMS_HOME?`