docs: add warnings/best practices for $myApi within useAsyncData

[johannschopplich]

🔗 Linked issue

#137

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSDoc annotations)
• 🐞 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

📝 Checklist

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

[johannschopplich]

@mattmess1221 What do you say about that addition to the docs?

[mattmess1221]

LGTM