docs: Add notes about NEXT_MANUAL_SIG_HANDLE (#43686)

It seems that setting `NEXT_MANUAL_SIG_HANDLE` in `.env` file is not working, so I added some notes.

## 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)

docs/deployment.md

@@ -136,7 +136,18 @@ Next.js will automatically load the latest version of your application in the ba
 
 Sometimes you might want to run some cleanup code on process signals like `SIGTERM` or `SIGINT`.
 
-You can do that by setting the env variable `NEXT_MANUAL_SIG_HANDLE` to `true` and then register a handler for that signal inside your `_document.js` file.
+You can do that by setting the env variable `NEXT_MANUAL_SIG_HANDLE` to `true` and then register a handler for that signal inside your `_document.js` file. Please note that you need to register env variable directly in the system env variable, not in the `.env` file.
+
+```json
+// package.json
+{
+  "scripts": {
+    "dev": "NEXT_MANUAL_SIG_HANDLE=true next dev",
+    "build": "next build",
+    "start": "NEXT_MANUAL_SIG_HANDLE=true next start"
+  }
+}
+```
 
 ```js
 // pages/_document.js