Document making AVFormatContext.packet_size unsigned API change.

Originally committed as revision 19251 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -12,6 +12,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2009-06-22 - r19250 - lavf 52.34.1 - AVFormatContext.packet_size
+  This is now an unsigned int instead of a signed int.
+
 2009-06-19 - r19222 - lavc 52.32.0 - AVSubtitle.pts
   Added a pts field to AVSubtitle which gives the subtitle's packet
   PTS in AV_TIME_BASE. Some subtitle de-/encoders (e.g. XSUB) will