add a fping6 known-issue note to documentation based on ZBX-18467 report

There is a technical details in this comment:
https://support.zabbix.com/browse/ZBX-18467#comment-608017

Based on the comment, as this issue still may be valid for quite long time, before we hopefully fix this fping6's issue in zabbix code (detect and workaround it), I'd add this note on the Known issues page:
https://www.zabbix.com/documentation/current/en/manual/installation/known_issues#simple-checks

If OS kernel is configured with IPv6 support disabled, fping6 binary cannot be used and produces unexpected error, like missing SUID bit set.
To workaround this, configure zabbix server/proxy to not be able to find the binary by defining and setting Fping6Location parameter to an empty value.