[Website]: Clarify how net.ipv4.tcp_retries2 setting is related with node-left on the <Troubleshoot an unstable cluster> document

[bojian-ben-li]

Type of issue

Missing information

What documentation page is affected

https://www.elastic.co/docs/troubleshoot/elasticsearch/troubleshooting-unstable-cluster

What happened?

Add clarification on how net.ipv4.tcp_retries2 setting is related with node-left on the document. Refer this discussion:
https://elastic.slack.com/archives/C045SG0FRGW/p1754056950088939

Additional info

No response

[bojian-ben-li]

I created a KB article for the content before they are updated in the document:
https://support.elastic.dev/knowledge/view/653db1e0