Update middleware documentation for custom server (#33535)

Upgrading from 12.0.0 to 12.0.8 results in `[dev] error - Error: To use middleware you must provide a `hostname` and `port` to the Next.js Server` and this requirement did not seem to be documented anywhere. Fixes #33450 which seems to have been erroneously closed.



## Bug

- [X] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `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`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`


Co-authored-by: JJ Kasper <[email protected]>

Author: saevarb

docs/middleware.md

@@ -76,6 +76,18 @@ Middleware runs directly after `redirects` and `headers`, before the first files
 
 Middleware uses a [strict runtime](/docs/api-reference/edge-runtime.md) that supports standard Web APIs like `fetch`. This works out of the box using `next start`, as well as on Edge platforms like Vercel, which use [Edge Functions](http://www.vercel.com/edge).
 
+## Custom Server
+
+When using a custom server with middleware, you must specify the hostname and port when instantiating your `NextApp`.
+
+```ts
+import next from 'next'
+// ...
+const port = process.env.PORT ? +process.env.PORT : 3000
+const dev = process.env.NODE_ENV !== 'production'
+const app = next({ dev, customServer: true, hostname: 'localhost', port })
+```
+
 ## Related
 
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