docs: add note about useFetch auto generated key

[victorkwok97]

🔗 Linked issue

#5895
#4955

❓ 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

Documentation about how the default key is generated was not updated.

📝 Checklist

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

[netlify]

✅ Deploy Preview for nuxt3-docs ready!

Name	Link
🔨 Latest commit	9f911c8
🔍 Latest deploy log	https://app.netlify.com/sites/nuxt3-docs/deploys/630db079c1540b00088cd249
😎 Deploy Preview	https://deploy-preview-7044--nuxt3-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[danielroe]

(We will likely need to pend merging this PR depending on what @pi0 decides to do with #6632.)

[pi0]

@danielroe Regardless we need to document behavior of of auto generated keys and it affects both asyncData and fetch. #6632 is effectively reverting auto key generation to previous code I've written that generated key based on URL. We can update docs by then to show this warning only about asyncData :)

[pi0]

(we shall update docs if changing behavior for next rc)