Update next build docs to include the --profile argument (#15766)

Co-authored-by: NTim Neutkens <tim@timneutkens.nl>
Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

docs/api-reference/cli.md

@@ -48,6 +48,14 @@ NODE_OPTIONS='--inspect' next
 
 The first load is colored green, yellow, or red. Aim for green for performant applications.
 
+You can enable production profiling for React with the `--profile` flag in `next build`. This requires Next.js 9.5:
+
+```bash
+next build --profile
+```
+
+After that, you can use the profiler in the same way as you would in development.
+
 ## Development
 
 `next dev` starts the application in development mode with hot-code reloading, error reporting, and more: