docs: note PPR default in cacheComponents reference (#92378)
### What?
Add a **Good to know** callout on the
[`cacheComponents`](https://nextjs.org/docs/app/api-reference/config/next-config-js/cacheComponents)
API reference, placed after the Usage list and before **Navigation with
Activity**.
### Why?
Developers enabling `cacheComponents: true` may see routes built as
partial prerenders (`◐ (Partial Prerender)`) without ever turning on a
separate PPR flag. The Caching getting-started guide already explains
PPR as the default with Cache Components; this surfaces the same idea on
the config page people often open first, and points to migration context
for Next.js 15 experimental PPR.
### How?
Single callout: PPR is used with the flag, legacy `experimental.ppr` /
`experimental_ppr` are superseded, `◐` in `next build` is expected, with
links to [How rendering
works](https://nextjs.org/docs/app/getting-started/caching#how-rendering-works)
and [Partial Prerendering
(PPR)](https://nextjs.org/docs/app/guides/upgrading/version-16#partial-prerendering-ppr)
in the Version 16 upgrade guide.
## PR checklist (Improving Documentation)
- Ran Prettier via lint-staged on commit; run `pnpm prettier-fix`
locally if CI flags anything.
- Follow the docs contribution guide:
https://nextjs.org/docs/community/contribution-guide
Made with [Cursor](https://cursor.com)
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Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
