doc/filters: add two lut2 examples

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

doc/filters.texi

@@ -9362,6 +9362,22 @@ The second input video bit depth.
 
 All expressions default to "x".
 
+@subsection Examples
+
+@itemize
+@item
+Highlight differences between two RGB video streams:
+@example
+lut2='ifnot(x-y,0,pow(2,bdx)-1):ifnot(x-y,0,pow(2,bdx)-1):ifnot(x-y,0,pow(2,bdx)-1)'
+@end example
+
+@item
+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
+@end itemize
+
 @section maskedclamp
 
 Clamp the first input stream with the second input and third input stream.