docs: warn that useState should only contain JSON-serializable content

[danielroe]

🔗 Linked issue

resolves nuxt/nuxt#14326.

❓ 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

Adds a brief warning about serializable content in useState. I haven't added similar warnings to data fetching composables as this is less likely to be an issue given that is originally coming over the wire as JSON, most likely, but I may change my mind, and happy to be convinced otherwise.

📝 Checklist

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

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[pi0]

@danielroe Do you mind to simplify the explanation to be similar to the guide page? #5994 (comment)