Improved cross-referencing and linking

xml:id is used to assign unique ID to elements which are target of link

almost any inline element can be used as a source of link

DocBook V4.5:

<section id="dir">
  <title>DIR command</title>
  <para>...</para>
</section>

<section id="ls">
  <title>LS command</title>
  <para>This command is a synonym for 
    <link linkend="dir"><command>DIR</command></link> command.</para>
</section>

DocBook V5.0:

<section xml:id="dir">
  <title>DIR command</title>
  <para>...</para>
</section>

<section xml:id="ls">
  <title>LS command</title>
  <para>This command is a synonym for 
    <command linkend="dir">DIR</command> command.</para>
</section>