Clear up rewrites documentation (#39238)
## Documentation / Examples
This PR specifically calls out in more clear language how the rewrites function behaves. This tripped me up recently as I recently came looking for more information, and the documentation left me with more questions than answers. This is an attempt to clear up the confusion I walked away with for anyone who comes in after me.
- [x] Make sure the linting passes by running `pnpm lint`
- [x] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
