doc/builders: add ids to the trivial builder definitions

This makes it possible to reference single function definitions,
for pointing people to their exact definition.

doc/builders/trivial-builders.xml

@@ -7,7 +7,7 @@
   Nixpkgs provides a couple of functions that help with building derivations. The most important one, <function>stdenv.mkDerivation</function>, has already been documented above. The following functions wrap <function>stdenv.mkDerivation</function>, making it easier to use in certain cases.
  </para>
  <variablelist>
-  <varlistentry>
+  <varlistentry xml:id="trivial-builder-runCommand">
    <term>
     <literal>runCommand</literal>
    </term>
@@ -40,7 +40,7 @@
 </programlisting>
    </listitem>
   </varlistentry>
-  <varlistentry>
+  <varlistentry xml:id="trivial-builder-runCommandCC">
    <term>
     <literal>runCommandCC</literal>
    </term>
@@ -50,7 +50,7 @@
     </para>
    </listitem>
   </varlistentry>
-  <varlistentry>
+  <varlistentry xml:id="trivial-builder-runCommandLocal">
    <term>
     <literal>runCommandLocal</literal>, <literal>runCommandCCLocal</literal>
    </term>
@@ -63,7 +63,7 @@
     </para></note>
    </listitem>
   </varlistentry>
-  <varlistentry>
+  <varlistentry xml:id="trivial-builder-writeText">
    <term>
     <literal>writeTextFile</literal>, <literal>writeText</literal>, <literal>writeTextDir</literal>, <literal>writeScript</literal>, <literal>writeScriptBin</literal>
    </term>
@@ -76,7 +76,7 @@
     </para>
    </listitem>
   </varlistentry>
-  <varlistentry>
+  <varlistentry xml:id="trivial-builder-symlinkJoin">
    <term>
     <literal>symlinkJoin</literal>
    </term>