explain litho transmittance better

plugins/ImageReader/ConfigUI.qml

@@ -168,7 +168,7 @@ UM.Dialog
         UM.TooltipArea {
             Layout.fillWidth:true
             height: childrenRect.height
-            text: catalog.i18nc("@info:tooltip","The percentage of light penetrating a print with a thickness of 1 millimeter.")
+            text: catalog.i18nc("@info:tooltip","The percentage of light penetrating a print with a thickness of 1 millimeter. Lowering this value increases the contrast in dark regions and decreases the contrast in light regions of the image.")
             Row {
                 width: parent.width