docs: update $fetch documentation

[ozum]

It is confusing why data is fetched twice when using $fetch on the client side without wrapping it with useAsyncData. It is explained here clearly. This PR updates docs to add clarification.

🔗 Linked issue

#18731

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

It is confusing why $fetch fetches data twice when used on the client side with SSR. This PR adds details about this behaviour.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[devzom]

Great one, bump, thanks! :)

[pi0]

We probably need to recommand and document context aware event.$fetch / useEventFetch() over directly $fetch inside the docs as this current version lacks the ability to automatically send headers (feature comparable to http/axios alternatives)

[Atinux]

I don't see any reference of useEventFetch(), is this something you are imagining @pi0 ?

Also, event.$fetch is for server routes right?