examples/minimal-configuration: improve comment

examples/minimal-configuration.nix

@@ -15,7 +15,8 @@
     name = "main"; # Set this to your system's main user
   };
 
-  # The system's main unprivileged user
+  # The system's main unprivileged user. This setting is usually part of your
+  # existing NixOS configuration.
   users.users.main = {
     isNormalUser = true;
     password = "a";