docs: clarify ensureStatic levels in the static guide
From vercel/shop dogfooding:
- explain why 'shell' is not enough for the product page with prefetch={true}
- explain the fallback shell on the first prefetch of products not returned
from generateStaticParams()
- say that ensureStatic does not move code, so stage API calls stay where
work must run later
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
