docs/runNixOS: configuration now supports configuration objects

Sounds obvious when you put it like that, but these attrsets returned
by NixOS didn't have a name before.

docs/modules/ROOT/pages/reference/nix-functions/runNixOS.adoc

@@ -23,11 +23,13 @@ When this option is set, xref:param-system[], xref:param-configuration[] and xre
 [[param-configuration]]
 === `configuration`
 
-A NixOS configuration file or module.
+A NixOS configuration or module.
 
 Example: `configuration = ./configuration.nix;`
 
-Required, unless you invoke the module system yourself via xref:param-config.
+Example (Flakes): `configuration = self.nixosConfigurations.foo;`
+
+Required, unless you set xref:param-config, which is not the preferred solution.
 
 
 [[param-nixpkgs]]