lavu/opt: document av_opt_copy function

libavutil/opt.h

@@ -815,6 +815,16 @@ void av_opt_freep_ranges(AVOptionRanges **ranges);
  */
 int av_opt_query_ranges(AVOptionRanges **, void *obj, const char *key, int flags);
 
+/**
+ * Copy options from src object into dest object.
+ *
+ * Options that require memory allocation (e.g. string or binary) are malloc'ed in dest object.
+ * Original memory allocated for such options is freed unless both src and dest options points to the same memory.
+ *
+ * @param dest Object to copy from
+ * @param src  Object to copy into
+ * @return 0 on success, negative on error
+ */
 int av_opt_copy(void *dest, void *src);
 
 /**