docs(cache): add example usage for cache.varies

[adamdehaven]

🔗 Linked issue

❓ 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

The cache.varies option for routeRules needs to be better documented to help understand the cache behavior and discarded headers, especially related to multi-tenant implementations.

The explanation is not currently obvious when reading through the docs; however, was better explained in nuxt/nuxt#26102 (comment) by @pi0. This PR surfaces the explanation and provides docs visitors a keyword of "tenant" they can search for as a hint. (It took me 3 days to figure out why my host headers were returning localhost 😅 )

I also created a corresponding PR for the Nuxt docs: nuxt/nuxt#26197

📝 Checklist

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

[pi0]

Thanks!