docs(router.md) Add note about page state and navigation (#26320)

This adds a note to the router doc explaining that page state is not reset by default when updating the URL but staying on the same page as this has come up in a few issues. 


## Documentation / Examples

- [x] Make sure the linting passes

x-ref: #26270

Author: ijjk

docs/api-reference/next/router.md

@@ -112,6 +112,8 @@ export default function Page() {
 }
 ```
 
+> **Note:** When navigating to the same page in Next.js, the page's state **will not** be reset by default, as the top-level React component is the same. You can manually ensure the state is updated using `useEffect`.
+
 Redirecting the user to `pages/login.js`, useful for pages behind [authentication](/docs/authentication):
 
 ```jsx