Add comment for the fix of attenuation.

Signed-off-by: Zheheng Liu <zheheng.liu@esrf.fr>

4_grains/gtGrainComputeBeamAttenuation.m

@@ -119,7 +119,8 @@ function [atts_tot, atts, abs_vol] = gtGrainComputeBeamAttenuation(gr, p, det_in
         end
     end
 
-    % Might need better scaling, for meaningful attenuation!! (0.1 -> mm to cm)
+    % It assumes the values to be in "[pixel size] ^ -1" instead of the
+    % classic "[cm] ^ -1"
     atts_tot = sum(atts, 2);
     atts_tot = exp(-atts_tot);