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