Manual: Add section on how to do rollbacks

doc/manual/troubleshooting.xml

@@ -81,4 +81,50 @@ just exit from the rescue shell.</para>
 </section>
 
 
+<section>
+
+<title>Rolling back configuration changes</title>
+
+<para>After running <command>nixos-rebuild</command> to switch to a
+new configuration, you may find that the new configuration doesn’t
+work very well.  In that case, there are several ways to return to a
+previous configuration.</para>
+
+<para>First, the GRUB boot manager allows you to boot into any
+previous configuration that hasn’t been garbage-collected.  These
+configurations can be found under the GRUB submenu “NixOS - All
+configurations”.  This is especially useful if the new configuration
+fails to boot.  After the system has booted, you can make the selected
+configuration the default for subsequent boots:
+
+<screen>
+$ /run/current-system/bin/switch-to-configuration boot</screen>
+
+</para>
+
+<para>Second, you can switch to the previous configuration in a running
+system:
+
+<screen>
+$ nixos-rebuild switch --rollback</screen>
+
+This is equivalent to running:
+
+<screen>
+$ /nix/var/nix/profiles/system-<replaceable>N</replaceable>-link/bin/switch-to-configuration switch</screen>
+
+where <replaceable>N</replaceable> is the number of the NixOS system
+configuration.  To get a list of the available configurations, do:
+
+<screen>
+$ ls -l /nix/var/nix/profiles/system-*-link
+<replaceable>...</replaceable>
+lrwxrwxrwx 1 root root 78 Aug 12 13:54 /nix/var/nix/profiles/system-268-link -> /nix/store/202b...-nixos-13.07pre4932_5a676e4-4be1055
+</screen>
+
+</para>
+
+</section>
+
+
 </chapter>