Add a way to pin a NixOS version within the module system.

This modification add a way to re-evaluate the module system with a
different version of NixOS, or with a different set of arguments.

nixos/doc/manual/configuration/configuration.xml

@@ -26,6 +26,7 @@ effect after you run <command>nixos-rebuild</command>.</para>
 
 <!-- FIXME: auto-include NixOS module docs -->
 <xi:include href="postgresql.xml" />
+<xi:include href="nixos.xml" />
 
 <!-- Apache; libvirtd virtualisation -->
 

nixos/doc/manual/default.nix

@@ -55,6 +55,7 @@ let
       cp -prd $sources/* . # */
       chmod -R u+w .
       cp ${../../modules/services/databases/postgresql.xml} configuration/postgresql.xml
+      cp ${../../modules/misc/nixos.xml} configuration/nixos.xml
       ln -s ${optionsDocBook} options-db.xml
       echo "${version}" > version
     '';

nixos/doc/manual/release-notes/rl-unstable.xml

@@ -6,6 +6,26 @@
 
 <title>Unstable</title>
 
+<para>In addition to numerous new and upgraded packages, this release
+has the following highlights:</para>
+
+<itemizedlist>
+
+  <listitem>
+    <para>You can now pin a specific version of NixOS in your <filename>configuration.nix</filename>
+    by setting:
+
+<programlisting>
+nixos.path = ./nixpkgs-unstable-2015-12-06/nixos;
+</programlisting>
+
+    This will make NixOS re-evaluate your configuration with the modules of
+    the specified NixOS version at the given path.  For more details, see
+    <xref linkend="module-misc-nixos" /></para>
+  </listitem>
+
+</itemizedlist>
+
 <para>When upgrading from a previous release, please be aware of the
 following incompatible changes:</para>