Restructuring the Nix manual

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diff --git a/doc/manual/packages/garbage-collection.xml b/doc/manual/packages/garbage-collection.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-garbage-collection'>
+
+<title>Garbage Collection</title>
+
+<para><command>nix-env</command> operations such as upgrades
+(<option>-u</option>) and uninstall (<option>-e</option>) never
+actually delete packages from the system.  All they do (as shown
+above) is to create a new user environment that no longer contains
+symlinks to the “deleted” packages.</para>
+
+<para>Of course, since disk space is not infinite, unused packages
+should be removed at some point.  You can do this by running the Nix
+garbage collector.  It will remove from the Nix store any package
+not used (directly or indirectly) by any generation of any
+profile.</para>
+
+<para>Note however that as long as old generations reference a
+package, it will not be deleted.  After all, we wouldn’t be able to
+do a rollback otherwise.  So in order for garbage collection to be
+effective, you should also delete (some) old generations.  Of course,
+this should only be done if you are certain that you will not need to
+roll back.</para>
+
+<para>To delete all old (non-current) generations of your current
+profile:
+
+<screen>
+$ nix-env --delete-generations old</screen>
+
+Instead of <literal>old</literal> you can also specify a list of
+generations, e.g.,
+
+<screen>
+$ nix-env --delete-generations 10 11 14</screen>
+
+</para>
+
+<para>After removing appropriate old generations you can run the
+garbage collector as follows:
+
+<screen>
+$ nix-store --gc</screen>
+
+If you are feeling uncertain, you can also first view what files would
+be deleted:
+
+<screen>
+$ nix-store --gc --print-dead</screen>
+
+Likewise, the option <option>--print-live</option> will show the paths
+that <emphasis>won’t</emphasis> be deleted.</para>
+
+<para>There is also a convenient little utility
+<command>nix-collect-garbage</command>, which when invoked with the
+<option>-d</option> (<option>--delete-old</option>) switch deletes all
+old generations of all profiles in
+<filename>/nix/var/nix/profiles</filename>.  So
+
+<screen>
+$ nix-collect-garbage -d</screen>
+
+is a quick and easy way to clean up your system.</para>
+
+<xi:include href="garbage-collector-roots.xml" />
+
+</chapter>
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