fix/7098 useOptimistic document

[AkashJana18]

Fixes #7098
This pull request updates the documentation for the useOptimistic hook to clarify the optional nature of the updateFn parameter.
Also added comments in example code for better understanding.

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Name	Status	Preview	Comments	Updated (UTC)
19-react-dev	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 16, 2024 1:21pm
react-dev	✅ Ready (Inspect)	Visit Preview		Aug 16, 2024 1:21pm

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Size changes

📦 Next.js Bundle Analysis for react-dev

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This PR introduced no changes to the JavaScript bundle! 🙌

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