Document env var expansion details

[bgrant0607]

It's briefly mentioned here:
https://kubernetes.io/docs/tasks/inject-data-application/define-command-argument-container/#use-environment-variables-to-define-arguments

But the expansion also works in env definitions.

And it is not mentioned here:
https://kubernetes.io/docs/tasks/inject-data-application/define-environment-variable-container/

There are also a number of details that should be documented in a reference page:
https://github.com/kubernetes/community/blob/master/contributors/design-proposals/node/expansion.md

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[bgrant0607]

/reopen

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@bgrant0607: Reopened this issue.

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/reopen

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[bgrant0607]

/remove-lifecycle rotten

[bgrant0607]

We still need a reference page with the details

[lucperkins]

@bgrant0607 I would suggest submitting a PR as this is a pretty obscure topic. I explored it a bit a while back and quickly realized that I’m ill equipped.

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