Clarify comment that explains why this header lacks multiple inclusion guards.

Originally committed as revision 10770 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/imgconvert_template.h

@@ -19,8 +19,9 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-/* This header has no multiple inclusion guards as it gets
- * included multiple times. */
+/* This header intentionally has no multiple inclusion guards. It is meant to
+ * be included multiple times and generates different code depending on the
+ * value of certain #defines. */
 
 #ifndef RGB_OUT
 #define RGB_OUT(d, r, g, b) RGBA_OUT(d, r, g, b, 0xff)