docs: correct ensureStatic behavior without Partial Prefetching
- 'shell' warns and has no effect, 'prefetch' adds no requirement
- describe how 'shell' and 'prefetch' handle request-specific work inside
and outside Suspense
- link the per-subtree controls from both level sections
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
