Add 'third_party/nix/' from commit 'be66c7a6b24e3c3c6157fd37b86c7203d14acf10' r/724

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diff --git a/third_party/nix/doc/manual/packages/channels.xml b/third_party/nix/doc/manual/packages/channels.xml
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+<chapter xmlns="http://docbook.org/ns/docbook"
+      xmlns:xlink="http://www.w3.org/1999/xlink"
+      xmlns:xi="http://www.w3.org/2001/XInclude"
+      version="5.0"
+      xml:id="sec-channels">
+
+<title>Channels</title>
+
+<para>If you want to stay up to date with a set of packages, it’s not
+very convenient to manually download the latest set of Nix expressions
+for those packages and upgrade using <command>nix-env</command>.
+Fortunately, there’s a better way: <emphasis>Nix
+channels</emphasis>.</para>
+
+<para>A Nix channel is just a URL that points to a place that contains
+a set of Nix expressions and a manifest.  Using the command <link
+linkend="sec-nix-channel"><command>nix-channel</command></link> you
+can automatically stay up to date with whatever is available at that
+URL.</para>
+
+<para>You can “subscribe” to a channel using
+<command>nix-channel --add</command>, e.g.,
+
+<screen>
+$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable</screen>
+
+subscribes you to a channel that always contains that latest version
+of the Nix Packages collection.  (Subscribing really just means that
+the URL is added to the file <filename>~/.nix-channels</filename>,
+where it is read by subsequent calls to <command>nix-channel
+--update</command>.) You can “unsubscribe” using <command>nix-channel
+--remove</command>:
+
+<screen>
+$ nix-channel --remove nixpkgs
+</screen>
+</para>
+
+<para>To obtain the latest Nix expressions available in a channel, do
+
+<screen>
+$ nix-channel --update</screen>
+
+This downloads and unpacks the Nix expressions in every channel
+(downloaded from <literal><replaceable>url</replaceable>/nixexprs.tar.bz2</literal>).
+It also makes the union of each channel’s Nix expressions available by
+default to <command>nix-env</command> operations (via the symlink
+<filename>~/.nix-defexpr/channels</filename>).  Consequently, you can
+then say
+
+<screen>
+$ nix-env -u</screen>
+
+to upgrade all packages in your profile to the latest versions
+available in the subscribed channels.</para>
+
+</chapter>