decoder/dsdiff: add documentation

doc/user.xml

@@ -737,6 +737,14 @@ cd mpd-version</programlisting>
       <title>Decoder plugins</title>
 
       <section>
+        <title><varname>dsdiff</varname></title>
+
+        <para>
+          Decodes DFF files containing DSDIFF data (e.g. SACD rips).
+        </para>
+      </section>
+
+      <section>
         <title><varname>mikmod</varname></title>
 
         <para>

src/decoder/dsdiff_decoder_plugin.c

@@ -17,6 +17,13 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+/* \file
+ *
+ * This plugin decodes DSDIFF data (SACD) embedded in DFF files.  It
+ * was modeled after the specification found here:
+ * http://www.sonicstudio.com/pdf/dsd/DSDIFF_1.5_Spec.pdf
+ */
+
 #include "config.h"
 #include "dsdiff_decoder_plugin.h"
 #include "decoder_api.h"