doc/filters.texi: add yet another laplacian edge detector

doc/filters.texi

@@ -5880,6 +5880,12 @@ Apply edge detect:
 convolution="0 1 0 1 -4 1 0 1 0:0 1 0 1 -4 1 0 1 0:0 1 0 1 -4 1 0 1 0:0 1 0 1 -4 1 0 1 0:5:5:5:1:0:128:128:128"
 @end example
 
+@item
+Apply laplacian edge detector which includes diagonals:
+@example
+convolution="1 1 1 1 -8 1 1 1 1:1 1 1 1 -8 1 1 1 1:1 1 1 1 -8 1 1 1 1:1 1 1 1 -8 1 1 1 1:5:5:5:1:0:128:128:0"
+@end example
+
 @item
 Apply emboss:
 @example