doc: Document the order in which avoid_negative_ts is applied compared to other timestamp options

Also try to document the rational behind it.

This might fix Ticket2376
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffmpeg-formats.texi

@@ -146,6 +146,10 @@ 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.
+When avoid_negative_ts is used together with copyts, vsync or async then
+avoid_negative_ts is applied last. This is neccessary so that containers
+which do not support negative timestamps wont get any and can be used
+with vsync/async and copyts.
 @end table
 
 @c man end FORMAT OPTIONS