docs: add client side only hint for app:beforeMount

[ppodds]

🔗 Linked issue

nuxt/nuxt#14488

❓ 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

Add client-side only hint for app:beforeMount hook. Resolves nuxt/nuxt#14488

📝 Checklist

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

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[atinux]

Damn, seems we worked at the same moment @ppodds 😅

I opened #6329