docs: correct io() prefetch behavior vs connection() (#101031)
## Summary
The `io()` reference said `connection()` blocks prefetches while `io()`
does not. Outside a cache scope they behave the same: `io()` returns a
hanging promise in `prerender`, `prerender-client`, and
`prerender-runtime`, like `connection()`, so the code after it is
excluded from build-time prerenders and from prefetches. The actual
difference is that `io()` resolves inside `"use cache"` scopes (and
works in Client Components), while `connection()` throws there.
- `io.mdx`: rewrite "How `io()` differs from `connection()`" around the
cache-scope difference.
- `connection.mdx`: fix the Good to know bullet that said `io()` "can
also be cached and prefetched".
- `errors/blocking-prerender-{crypto,current-time,random}.mdx`: drop
"`io()` does not block prefetches".
## Verification
Manual, with a production build of 16.5.0-canary.27 and a Playwright
probe that held navigation requests so only prefetched data could
render:
- With `partialPrefetching: true`, content after `await io()` outside a
cache scope is not prefetched (fallback shows), same as `connection()`.
A runtime prefetch on a route that reads `cookies()` still stops at
`io()`.
- A helper that calls `io()` produces a fresh value per request in
uncached code and a build-time value (static route) when called from a
`"use cache"` function. The `connection()` equivalent fails the build.
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