Add missing itemimage attribute to playtree doc

doc/skins/skins2-howto.xml

@@ -744,6 +744,10 @@ difficulty to understand how VLC skins work.</para>
     <para>Boolean to indicate whether the playlist should use the tree structure or be completely "flat" (only show the leafs of the tree).</para>
     <para>A flat playtree will work like old-style playlists.</para>
     <para>Default value: false</para>
+  </sect4>
+  <sect4 id="itemimage">
+    <title>itemimage</title>
+    <para>Identifiant of a <link linkend="Bitmap">Bitmap</link> shown to the left of a leaf (playlist item).</para>
  </sect4>
   <sect4 id="openimage">
     <title>openimage</title>