AVOptions: move doxy for av_opt_set_defaults() from opt.c to opt.h

Also change it to be more readable.

libavutil/opt.c

@@ -414,12 +414,6 @@ int av_opt_show2(void *obj, void *av_log_obj, int req_flags, int rej_flags)
     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_defaults2(void *s, int mask, int flags)
 {
     const AVOption *opt = NULL;

libavutil/opt.h

@@ -160,6 +160,11 @@ const AVOption *av_next_option(void *obj, const AVOption *last);
  */
 int av_opt_show2(void *obj, void *av_log_obj, int req_flags, int rej_flags);
 
+/**
+ * Set the values of all AVOption fields to their default values.
+ *
+ * @param s an AVOption-enabled struct (its first member must be a pointer to AVClass)
+ */
 void av_opt_set_defaults(void *s);
 void av_opt_set_defaults2(void *s, int mask, int flags);