adding note that edge api routes are not supported with ISR (#43572)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ]
[e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Co-authored-by: JJ Kasper <jj@jjsweb.site>

docs/basic-features/data-fetching/incremental-static-regeneration.md

@@ -27,6 +27,8 @@ description: 'Learn how to create or update static pages at runtime with Increme
 
 Next.js allows you to create or update static pages _after_ you’ve built your site. Incremental Static Regeneration (ISR) enables you to use static-generation on a per-page basis, **without needing to rebuild the entire site**. With ISR, you can retain the benefits of static while scaling to millions of pages.
 
+> Note: the `experimental-edge` runtime (https://nextjs.org/docs/api-reference/edge-runtime) is currently not compatible with ISR although can leverage `stale-while-revalidate` by setting the `cache-control` header manually.
+
 To use ISR, add the `revalidate` prop to `getStaticProps`:
 
 ```jsx