Update `k8s dumb-init` FF doc to convey its support in both Kubernetes modes
What does this MR do?
In !4485 (merged) we add FF_USE_DUMB_INIT_WITH_KUBERNETES_EXECUTOR support for the k8s exec mode. This change was not reflected in the doc.
Why was this MR needed?
To reflect the change made in !4485 (merged) in the documentation