Mention router.isPreview on Preview Mode page (#39830)
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 <fbae0e752deb2c0fed663fa6dec6c97ee2546cfe@users.noreply.github.com>
JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js