update unsupported items in Next.js preview

cc:// @craigshoemaker

Author: Reshmi-Sriram

includes/static-web-apps-nextjs-unsupported.md

@@ -19,4 +19,4 @@ During the preview, the following features of Static Web Apps are unsupported fo
   - Features such as custom headers and routing can be controlled using the `next.config.js` file.
 - `skip_app_build` and `skip_api_build` can't be used.
 - The maximum app size for the hybrid Next.js application is 100 MB. Consider using Static HTML exported Next.js apps if your requirement is more than 100 MB.
-- Incremental static regeneration (ISR) does not support caching images
+- Incremental static regeneration (ISR) does not support caching images and [On-demand revalidation](https://nextjs.org/docs/basic-features/data-fetching/incremental-static-regeneration#using-on-demand-revalidation)