[FDS] Improve seed docstring

[charlesbvll]

Issue

Description

The docstring for FederatedDataset didn't specify the default seed or what happened when seed=None.

Related issues/PRs

N/A

Proposal

Explanation

Add more details (based on https://huggingface.co/docs/datasets/v2.18.0/en/package_reference/main_classes#datasets.Dataset.shuffle).

Checklist

• Implement proposed change
• Update the changelog entry below
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Changelog entry

Any other comments?

N/A

[adam-narozniak]

Ok, we can have a partial copy of the docs from the numpy (HF also seemed to copy the docstring from numpy). I quickly looked at "entropy" in the numpy documentation, though I'm not really familiar with that in this context.
Anyway, it's fine with me.