Mention router.isPreview on Preview Mode page (#39830)

The documentation is concise and straightforward, so I just added a sentence.
Let me know if you'd like to describe more, add an example, etc.

Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

docs/advanced-features/preview-mode.md

@@ -173,6 +173,8 @@ https://<your-site>/api/preview?secret=<token>&slug=<path>
 
 ## More Details
 
+> **Note**: during rendering `next/router` exposes an `isPreview` flag, see the [router object docs](/docs/api-reference/next/router.md#router-object) for more info.
+
 ### Clear the Preview Mode cookies
 
 By default, no expiration date is set for Preview Mode cookies, so the preview session ends when the browser is closed.