Document avcodec_pix_fmt_to_codec_tag().

Originally committed as revision 21006 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -2984,7 +2984,13 @@ void avcodec_set_dimensions(AVCodecContext *s, int width, int height);
  * Finally if no pixel format has been found, returns PIX_FMT_NONE.
  */
 enum PixelFormat avcodec_get_pix_fmt(const char* name);
-unsigned int avcodec_pix_fmt_to_codec_tag(enum PixelFormat p);
+
+/**
+ * Returns a value representing the fourCC code associated to the
+ * pixel format pix_fmt, or 0 if no associated fourCC code can be
+ * found.
+ */
+unsigned int avcodec_pix_fmt_to_codec_tag(enum PixelFormat pix_fmt);
 
 #define FF_LOSS_RESOLUTION  0x0001 /**< loss due to resolution change */
 #define FF_LOSS_DEPTH       0x0002 /**< loss due to color depth change */