Add doxygen docs for av_xiphlacing

Patch by Josh Allmann (joshua allmann gmail com)

Originally committed as revision 22632 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -3784,6 +3784,13 @@ int av_picture_crop(AVPicture *dst, const AVPicture *src,
 int av_picture_pad(AVPicture *dst, const AVPicture *src, int height, int width, enum PixelFormat pix_fmt,
             int padtop, int padbottom, int padleft, int padright, int *color);
 
+/**
+ * Encodes extradata length to a buffer. Used by xiph codecs.
+ *
+ * @param s buffer to write to; must be at least (v/255+1) bytes long
+ * @param v size of extradata in bytes
+ * @return number of bytes written to the buffer.
+ */
 unsigned int av_xiphlacing(unsigned char *s, unsigned int v);
 
 /**