doc/filters: Slightly improve the smartblur documentation.

Fixes ticket #6034.

doc/filters.texi

@@ -12572,20 +12572,20 @@ in [-30,0] will filter edges. Default value is 0.
 @item chroma_radius, cr
 Set the chroma radius. The option value must be a float number in
 the range [0.1,5.0] that specifies the variance of the gaussian filter
-used to blur the image (slower if larger). Default value is 1.0.
+used to blur the image (slower if larger). Default value is @option{luma_radius}.
 
 @item chroma_strength, cs
 Set the chroma strength. The option value must be a float number
 in the range [-1.0,1.0] that configures the blurring. A value included
 in [0.0,1.0] will blur the image whereas a value included in
-[-1.0,0.0] will sharpen the image. Default value is 1.0.
+[-1.0,0.0] will sharpen the image. Default value is @option{luma_strength}.
 
 @item chroma_threshold, ct
 Set the chroma threshold used as a coefficient to determine
 whether a pixel should be blurred or not. The option value must be an
 integer in the range [-30,30]. A value of 0 will filter all the image,
 a value included in [0,30] will filter flat areas and a value included
-in [-30,0] will filter edges. Default value is 0.
+in [-30,0] will filter edges. Default value is @option{luma_threshold}.
 @end table
 
 If a chroma option is not explicitly set, the corresponding luma value