-async and -vsync documentation. Based on info from the mailinglist.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@6028 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

doc/ffmpeg-doc.texi

@@ -504,6 +504,16 @@ streams. This option is used for automatic FFserver testing.
 @item -loop_output number_of_times
 Repeatedly loop output for formats that support looping such as animated GIF
 (0 will loop the output infinitely).
+@item -vsync parameter
+Video sync method. Video will be stretched/sqeezed to match the timestamps,
+it is done by duplicating and droping frames. With -map you can select from
+which stream the timestamps should be taken, you can leave either video or
+audio unchanged and sync the remaining stream(s) to the unchanged one.
+@item -async samples_per_second
+Audio sync method. "Stretches/sqeezes" the audio stream to match the timestamps,
+the parameter is the maximum samples per second by which the audio is changed.
+-async 1 is a special case where only the start of the audio stream is corrected
+without any later correction.
 @end table
 
 @node FFmpeg formula evaluator