docs(staletime): add clarification around prefetch false

[samcx]

Why?

Add clarification around when the prefetch prop on the Link component is set to false.

• x-ref: https://github.com/vercel/next.js/issues/51788#issuecomment-2088521708
• https://github.com/vercel/next.js/blob/canary/test/e2e/app-dir/app-client-cache/client-cache.test.ts#L202

Closes NEXT-3275