Document the behavior of av_metadata_get() if the prev parameter is

NULL.

Originally committed as revision 22704 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avformat.h

@@ -131,6 +131,7 @@ typedef struct AVMetadataConv AVMetadataConv;
 /**
  * Gets a metadata element with matching key.
  * @param prev Set to the previous matching element to find the next.
+ *             If set to NULL the first matching element is returned.
  * @param flags Allows case as well as suffix-insensitive comparisons.
  * @return Found tag or NULL, changing key or value leads to undefined behavior.
  */