docs: Clarify manual graceful shutdown in dev server

docs/02-app/01-building-your-application/07-deploying/index.mdx

@@ -136,12 +136,14 @@ 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. Please note that you need to register env variable directly in the system env variable, not in the `.env` file.
+You can set 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 the env variable directly in the system env variable, not in the `.env` file.
+
+> **Note**: Manual signal handling is not available in `next dev`.
 
 ```json filename="package.json"
 {
   "scripts": {
-    "dev": "NEXT_MANUAL_SIG_HANDLE=true next dev",
+    "dev": "next dev",
     "build": "next build",
     "start": "NEXT_MANUAL_SIG_HANDLE=true next start"
   }