doc: try to improve avoid_negative_ts documentation

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffmpeg-formats.texi

@@ -142,6 +142,10 @@ Use wallclock as timestamps.
 @item avoid_negative_ts @var{integer} (@emph{output})
 Shift timestamps to make them positive. 1 enables, 0 disables, default
 of -1 enables when required by target format.
+When timestamps are shifted, all timestamps of a output file are shifted
+together by the same amount. AV sync, subtitle sync and relative timestamp
+differences are preserved compared to how they would have been without
+shifting.
 @end table
 
 @c man end FORMAT OPTIONS