adds doxygen docs to av_opt_set_defaults. Patch by Takis

Originally committed as revision 6226 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/opt.c

@@ -300,6 +300,12 @@ int av_opt_show(void *obj, void *av_log_obj){
     return 0;
 }
 
+/** Set the values of the AVCodecContext or AVFormatContext structure.
+ * They are set to the defaults specified in the according AVOption options
+ * array default_val field.
+ *
+ * @param s AVCodecContext or AVFormatContext for which the defaults will be set
+ */
 void av_opt_set_defaults(void *s)
 {
     AVOption *opt = NULL;