docs: clarify /_not-found failures and <html> attribute reads under Cache Components (#95163)
## Summary
When the root layout reads request data (`cookies()`, `headers()`) under
Cache Components, the validation error often surfaces on `/_not-found` —
a real prerendered route that inherits the root layout, even though the
user has no `not-found.tsx` file. This is consistently confusing
(#67532), so:
- `errors/blocking-prerender-runtime.mdx` and
`errors/blocking-prerender-dynamic.mdx`: added a gotcha bullet pointing
the fix at the root layout when the failing route is `/_not-found`.
- `docs/01-app/02-guides/migrating-to-cache-components.mdx`: added a
`Good to know` callout in the `cookies, headers, and searchParams`
section linking to the existing [Themes
recipe](/docs/app/guides/preventing-flash-before-hydration#themes) for
the `<html data-theme>` case, since attributes on the root element can
not be wrapped in `<Suspense>`.
## Verification
- `npx alex` clean on all three files.
<!-- NEXT_JS_LLM_PR -->
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Co-authored-by: Joseph Chamochumbi <joseph.chamochumbi@vercel.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
next.js
d4b23403 - docs: clarify /_not-found failures and <html> attribute reads under Cache Components (#95163)
