Expound PartialState docstring

[muellerzr]

What does this PR do?

These merge conflicts are getting fun, but it's nice to see more people wanting better uses of PartialState!

In this one we explicitly discuss how to use InitProcessGroupKwargs with the PartialState, which is needed if you want to modify the init outside the Accelerator

Closes #2587

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@BenjaminBossan

[HuggingFaceDocBuilderDev]

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[BenjaminBossan]

Thanks for expanding the docstring of PartialState.