docs: note Suspense requirement for connection() with Cache Components (#100810)
## Summary
The `connection()` API reference did not say that, with Cache
Components, the calling component needs a `<Suspense>` boundary above
it. The caching guide and the `blocking-prerender-dynamic` error page
both require one, and the `io()` and `navigation()` references show it.
Adds a Good to know bullet: the boundary can be direct or an ancestor
such as `loading.js`, the nearest fallback is prerendered into the
static shell, and without a boundary navigations block instead of being
instant.
## Verification
- Checked link targets: `loading`, glossary `#static-shell`,
`/docs/messages/blocking-prerender-dynamic`
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