docs(data-fetching): add note about difference between useFetch and useAsyncData

[danielroe]

🔗 Linked issue

https://github.com/nuxt/framework/discussions/3414

❓ 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

This adds a brief note to answer what the difference is b/t useFetch and useAsyncData, and switches the order of them in the docs.

📝 Checklist

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

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[pi0]

Really nice improvements 👍🏼 We might extend it a bit in the future, mentioning more additional features of useFetch...