configuration.nix: Document the stateVersion more

nixos/modules/installer/tools/nixos-generate-config.pl

@@ -618,8 +618,11 @@ $bootLoaderConfig
   #   uid = 1000;
   # };
 
-  # The NixOS release to be compatible with for stateful data such as databases.
-  system.stateVersion = "${\(qw(@nixosRelease@))}";
+  # This value determines the NixOS release with which your system is to be
+  # compatible, in order to avoid breaking some software such as database
+  # servers. You should change this only after NixOS release notes say you
+  # should.
+  system.stateVersion = "${\(qw(@nixosRelease@))}"; # Did you read the comment?
 
 }
 EOF