modularity: Document the ability to use non-files in imports

nixos/doc/manual/configuration/modularity.xml

@@ -127,4 +127,23 @@ nix-repl> map (x: x.hostName) config.<xref linkend="opt-services.httpd.virtualHo
 [ "example.org" "example.gov" ]
 </screen>
  </para>
+
+ <para>
+   While abstracting your configuration, you may find it useful to generate
+   modules using code, instead of writing files. This is also an option; the
+   below would have the same effect as importing a file which sets those
+   options.
+   <screen>
+     { config, pkgs, ... }:
+
+     let netConfig = { hostName }: {
+       networking.hostName = hostName;
+       networking.useDHCP = false;
+    };
+
+    in
+
+    { imports = [ (netConfig "nixos.localdomain") ]; }
+  </screen>
+</para>
 </section>