Document k8_watch_api feature flag

What does this MR do and why?

Adds documentation for the k8_watch_api feature flag introduced by !133734 (merged).

I thought about giving this its own topic to highlight the change, but because the Kubernetes API integrates with both the cluster status and the Flux reconciliation status, I opted to just add some version history to ## View a dashboard.

Related issues

#422945 (closed)