avutil/detection_bbox: Add doxy for missing argument

libavutil/detection_bbox.h

@@ -93,6 +93,7 @@ av_get_detection_bbox(const AVDetectionBBoxHeader *header, unsigned int idx)
  * AVDetectionBBox, and initializes the variables.
  * Can be freed with a normal av_free() call.
  *
+ * @param nb_bboxes number of AVDetectionBBox structures to allocate
  * @param out_size if non-NULL, the size in bytes of the resulting data array is
  * written here.
  */