doc/swscaler: explain default Lanczos parameter

Explain that the default Lanczos filter parameter is 3 and that it can be
changed by the param0 option.
Signed-off-by: Werner Robitza <werner.robitza@gmail.com>

doc/scaler.texi

@@ -48,7 +48,8 @@ Select Gaussian rescaling algorithm.
 Select sinc rescaling algorithm.
 
 @item lanczos
-Select Lanczos rescaling algorithm.
+Select Lanczos rescaling algorithm. The default width (alpha) is 3 and can be
+changed by setting @code{param0}.
 
 @item spline
 Select natural bicubic spline rescaling algorithm.