Clarify image change restart behavior for sidecar containers

[dshebib]

Clarify that only chaging the image of a sidecar container will not restart the pod.

Previously had contradicting information:

Altering an init container image field is equivalent to restarting the Pod.
and
The Pod will not be restarted when the init container image is changed,

xref: kubernetes/kubernetes#121333

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[dshebib]

@sftim included your comments

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PR needs rebase.

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[tengqm]

@dshebib Please help resolve the conflicts.