[doc/apt.conf.5.xml] mention the apt.conf.d dir

Patch from Vincent McIntyre, thanks! (Closes: #520831)

debian/changelog

@@ -8,6 +8,8 @@ apt (0.7.22.3) unstable; urgency=low
   * apt-pkg/contrib/strutl.cc:
     - enable thousand separator according to the current locale
       (by Luca Bruno, Closes: #223712)
+  * doc/apt.conf.5.xml:
+    - mention the apt.conf.d dir (by Vincent McIntyre, Closes: #520831)
 
   [ George Danchev ]
   * cmdline/apt-cache.cc:

doc/apt.conf.5.xml

@@ -822,7 +822,17 @@ is commented.
  </refsect1>
  
  <refsect1><title>Files</title>
-   <para><filename>/etc/apt/apt.conf</filename></para>
+   <variablelist>
+      <varlistentry><term><filename>/etc/apt/apt.conf</filename></term>
+      <listitem><para>APT configuration file.
+      Configuration Item: <literal>Dir::Etc::Main</literal>.</para></listitem> 
+      </varlistentry>
+
+      <varlistentry><term><filename>/etc/apt/apt.conf.d/</filename></term>
+      <listitem><para>APT configuration file fragments.
+      Configuration Item: <literal>Dir::Etc::Parts</literal>.</para></listitem>
+      </varlistentry>
+   </variablelist>
  </refsect1>
  
  <refsect1><title>See Also</title>