Update prefetching.mdx providing more clarity on the usage of `router.prefetch()` (#84903)
## What
Updated `prefetching.mdx` in the App Router documentation providing more
clarity on the usage of `router.prefetch()`.
## Why
We found the current section on `Manual Prefetching` to be lacking in
detail and a bit misleading as well.
The issue with the current version of the documentation is that the code
snippet provided:
```tsx
const router = useRouter();
router.prefetch();
```
Makes it seem like `router.prefetch()` can be called at the same level
as `useRouter()`, i.e. in the component body, even though that causes
the page to crash for many possible reasons, such as `window is not
defined`, or `Router action dispatched before initialization.`.
I am not sure if this is helpful/correct or not, just thought this may
help someone. If not feel free to close this PR. Thanks in advance for
your time.
## Reproduction
Here is a codesandbox reproducing the issue that occurs if you follow
the code snippet trivially:
https://codesandbox.io/p/devbox/s5pmsj
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Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>