minor modifs in response to Pulkomandy's remarks

src/buttons_effects.c

@@ -380,7 +380,7 @@ void Button_Constraint_menu(void)
           case IMAGE_MODE_RASTER:
           case IMAGE_MODE_THOMSON:
             Snap_width = 8;
-            Snap_height = 400;
+            Snap_height = 999;  // maximum value (3 digits)
             break;
           default:
             set_grid = 0;

src/graph.c

@@ -3477,7 +3477,7 @@ static void Pixel_in_screen_underlay_with_opt_preview(word x,word y,byte color,i
   }
 }
 
-/// Paint a single pixel in the layer 5 of mode 5
+/// Paint a single pixel in the layer 5 of CPC rasterized modes
 ///
 /// used when @ref IMAGE_MODE_MODE5 or @ref IMAGE_MODE_RASTER is active.
 ///