doc/filters: add yet another fftfilt example

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

doc/filters.texi

@@ -6468,6 +6468,12 @@ Sharpen:
 fftfilt=dc_Y=0:weight_Y='1+squish(1-(Y+X)/100)'
 @end example
 
+@item
+Blur:
+@example
+fftfilt=dc_Y=0:weight_Y='exp(-4 * ((Y+X)/(W+H)))'
+@end example
+
 @end itemize
 
 @section field