doc/filters.texi: add another lut2 example

Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -10004,6 +10004,12 @@ Highlight differences between two YUV video streams:
 @example
 lut2='ifnot(x-y,0,pow(2,bdx)-1):ifnot(x-y,pow(2,bdx-1),pow(2,bdx)-1):ifnot(x-y,pow(2,bdx-1),pow(2,bdx)-1)'
 @end example
+
+@item
+Show max difference between two video streams:
+@example
+lut2='if(lt(x,y),0,if(gt(x,y),pow(2,bdx)-1,pow(2,bdx-1))):if(lt(x,y),0,if(gt(x,y),pow(2,bdx)-1,pow(2,bdx-1))):if(lt(x,y),0,if(gt(x,y),pow(2,bdx)-1,pow(2,bdx-1)))'
+@end example
 @end itemize
 
 @section maskedclamp