manual: mention the "channel:" shorthand for NIX_PATH

Bumped to 15.09 because older channels, when downloaded from
nixos.org, require firefox to be accessed via `pkgs.firefox`

doc/manual/command-ref/env-common.xml

@@ -52,10 +52,15 @@ nixpkgs=/home/eelco/Dev/nixpkgs-branch:/etc/nixos</screen>
     <envar>NIX_PATH</envar> to
 
     <screen>
-nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-14.12.tar.gz</screen>
+nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-15.09.tar.gz</screen>
 
     tells Nix to download the latest revision in the Nixpkgs/NixOS
-    14.12 channel.</para>
+    15.09 channel.</para>
+
+    <para>A following shorthand can be used to refer to the official channels:
+
+    <screen>nixpkgs=channel:nixos-15.09</screen>
+    </para>
 
     <para>The search path can be extended using the <option
     linkend="opt-I">-I</option> option, which takes precedence over