Make troubleshooting an appendix

doc/manual/troubleshooting/collisions-nixenv.xml

-<chapter xmlns="http://docbook.org/ns/docbook"
+<section 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="ch-collisions-nixenv">
+      xml:id="sec-collisions-nixenv">
 
 <title>Collisions in <command>nix-env</command></title>
 
@@ -40,4 +40,4 @@ to implement some conflict resolution policy.  E.g., the script could
 be modified to rename conflicting file names, or to pick one over the
 other.</para>
 
-</chapter>
+</section>
\ No newline at end of file

doc/manual/troubleshooting/links-nix-store.xml

-<chapter xmlns="http://docbook.org/ns/docbook"
+<section 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="ch-links-nix-store">
+      xml:id="sec-links-nix-store">
 
 <title><quote>Too many links</quote> Error in the Nix store</title>
 
@@ -40,4 +40,4 @@ more than 32,000 subdirectories per directory, such as ReiserFS.
 ReiserFS lies to the kernel by reporting a link count of 1 if it
 exceeds the limit.)</para>
 
-</chapter>
+</section>
\ No newline at end of file

doc/manual/troubleshooting/troubleshooting.xml

-<part 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="part-troubleshooting">
+<appendix 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="ch-troubleshooting">
 
 <title>Troubleshooting</title>
 
-<partintro>
 <para>This section provides solutions for some common problems.  See
 the <link xlink:href="http://bugs.strategoxt.org/browse/NIX">Nix
 bug tracker</link> for a list of currently known issues.</para>
-</partintro>
 
 <xi:include href="collisions-nixenv.xml" />
 <xi:include href="links-nix-store.xml" />
 
-</part>
+</appendix>