avformat/avformat: Improve documentation of av_stream_get_side_data

Document that it also sets the size in case the desired side data is
absent (if the pointer has been supplied).

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/avformat.h

@@ -2189,7 +2189,8 @@ uint8_t *av_stream_new_side_data(AVStream *stream,
  *
  * @param stream stream
  * @param type desired side information type
- * @param size pointer for side information size to store (optional)
+ * @param size If supplied, *size will be set to the size of the side data
+ *             or to zero if the desired side data is not present.
  * @return pointer to data if present or NULL otherwise
  */
 uint8_t *av_stream_get_side_data(const AVStream *stream,