docs: warn about using different env variables for runtimeConfig

[manniL]

❓ 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

I've seen the pattern a few times, causing issues during runtime.

export default defineNuxtConfig({
  runtimeConfig: {
    apiSecret: process.env.SOME_OTHER_VALUE, // This will break when trying to set SOME_OTHER_VALUE during runtime
    apiKey: process.env.API_KEY.trim(), // This won't transform values when setting API_KEY during runtime
  },
})

📝 Checklist

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

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[danielroe]

👌