Added cross-references to NixOS manual

nixos/doc/manual/configuration/customizing-packages.xml

@@ -28,7 +28,7 @@ has a dependency on GTK+ 2.  If you want to build it against GTK+ 3,
 you can specify that as follows:
 
 <programlisting>
-environment.systemPackages = [ (pkgs.emacs.override { gtk = pkgs.gtk3; }) ];
+<xref linkend="opt-environment.systemPackages"/> = [ (pkgs.emacs.override { gtk = pkgs.gtk3; }) ];
 </programlisting>
 
 The function <varname>override</varname> performs the call to the Nix
@@ -38,7 +38,7 @@ the set of arguments specified by you.  So here the function argument
 causing Emacs to depend on GTK+ 3.  (The parentheses are necessary
 because in Nix, function application binds more weakly than list
 construction, so without them,
-<literal>environment.systemPackages</literal> would be a list with two
+<xref linkend="opt-environment.systemPackages"/> would be a list with two
 elements.)</para>
 
 <para>Even greater customisation is possible using the function
@@ -51,7 +51,7 @@ For instance, if you want to override the source code of Emacs, you
 can say:
 
 <programlisting>
-environment.systemPackages = [
+<xref linkend="opt-environment.systemPackages"/> = [
   (pkgs.emacs.overrideAttrs (oldAttrs: {
     name = "emacs-25.0-pre";
     src = /path/to/my/emacs/tree;