KEP-753: Update documentation to clarify restartPolicy behavior

[gjkim42]

• One-line PR description:

• Issue link: Sidecar Containers  #753

• Other comments:
  Desired behavior of a startup failed restartable init container in a pod with restartPolicy set to Never kubernetes#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

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[pacoxu]

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