IBFT has immediate finality

[siladu]

Pull request checklist

https://consensys.github.io/doc.goquorum/PR-232/concepts/consensus/comparing-poa/

Before creating the PR

Make sure that:

• You've read the contribution guidelines.
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Describe the change

Docs incorrectly stated that IBFT does not have immediate finality

Impacted parts

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• Documentation content
• Documentation page organization

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[alexandratran]

Thanks for catching this mistake! LGTM after minor stylistic suggestions.

[bgravenorst]

LGTM