Document av_probe_input_format2 API addition.

Originally committed as revision 23003 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -12,6 +12,10 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-05-01 - r23002 - lavf 52.62.0 - probe function
+  Add av_probe_input_format2 to API, it allows ignoring probe
+  results below given score and returns the actual probe score.
+
 2010-04-01 - r22806 - lavf 52.61.0 - metadata API
   Add a flag for av_metadata_set2() to disable overwriting of
   existing tags.