Add warning about backoffLimit limitation

[tengqm]

As per discussed in kubernetes/kubernetes#54870, the usage of the backoffLimit has some limitations.
This PR adds a Note for this.

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[weiwei04]

shall we emphasize the difference with OnFailure and Never ? for OnFailure, the failed Pod will restart on the same kubelet host, so emptyDir/hostPath is still there, for Never the failed Pod got delete and a replacement will create on another(may be the same) kubelet host, if the Pod use emptyDir/hostPath to store some tmp data, the tmp data may be lost.

[tengqm]

@weiwei04 Thanks for the suggestion. The detailed description about restart policies and the differences among them should be discussed here: https://kubernetes.io/docs/concepts/workloads/pods/pod-lifecycle/#restart-policy

Feel free to help out.

[steveperry-53]

@tengqm Are you ready for me to merge this? It looks like the discussion between you and @weiwei04 pertains to a different topic.

[tengqm]

Yes. @weiwei04 was suggesting some more improvements that can be left to a future PR.

[weiwei04]

Looks good to me, sorry for the late replay @steveperry-53 I agreewith @tengqm that can be left to a future PR in another part of the docs.