doc/filters: add one more blend example

This is faster than geq example below.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -4243,6 +4243,12 @@ Apply uncover up-left effect:
 blend=all_expr='if(gte(T*SH*40+Y,H)*gte((T*40*SW+X)*W/H,W),A,B)'
 @end example
 
+@item
+Split diagonally video and shows top and bottom layer on each side:
+@example
+blend=all_expr=if(gt(X,Y*(W/H)),A,B)
+@end example
+
 @item
 Display differences between the current and the previous frame:
 @example