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

NULL.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22704 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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.
  */