doc/filters/setpts: add random jitter generation example

doc/filters.texi

@@ -31061,6 +31061,12 @@ Set fixed rate of 25 frames per second:
 setpts=N/(25*TB)
 @end example
 
+@item
+Apply a random jitter effect of +/-100 TB units:
+@example
+setpts=PTS+randomi(0, -100\,100)
+@end example
+
 @item
 Set fixed rate 25 fps with some jitter:
 @example