Update Kafka settings for multiple instances documentation

docs currently state only the following: to set the <kafkaprop>.group.id property https://docs.opennms.org/opennms/releases/latest/guide-install/guide-install.html#kafka-setup under "Multiple OpenNMS Horizon Instances"

Need to set org.opennms.instance.id=OpenNMS-Stage as a key value/pair under the Feature Profile - General - System Properties. (Some background under NMS-13101)