KEP-753: Update documentation to clarify `restartPolicy` behavior

[gjkim42]

- One-line PR description:

• Issue link: #753

• Other comments: https://github.com/kubernetes/kubernetes/issues/120234

The implementation is different from the current documentation. This updates the documentation to clarify the relationship between the container-level restartPolicy and the pod-level restartPolicy.

We may introduce a new concept (or policy) for the restart behavior of the initialization.

/cc @SergeyKanzhelev

[pacoxu]

/lgtm if we do not want to change the behavior, we should sync the current behavior to the KEP.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gjkim42, mrunalp, SergeyKanzhelev

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