📖 improve docs of rescaffold

[Sijoma]

So I was irritated by reading the documentation and also the CLI --help output.

I was surprised that the output-dir actually defaults to output-dir. I also did not understand how I could use git to diff the files with upstream if they are created in a sub directory. I would understand how this works if the files would be regenerated in place.

I've adjusted the docs & command to be more explicit about the default value output-dir.
Furthermore, I've adjusted the note to explain that you can diff the local files against the output-dir (the folder that was configured by the user).

relates #4478

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @Sijoma. Thanks for your PR.

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