[FEM] task_material_common: add a comment

src/Mod/Fem/femtaskpanels/task_material_common.py

@@ -138,6 +138,8 @@ class _TaskPanel:
         self.toggleInputFieldsReadOnly()
 
         # hide some groupBox according to material category
+        # note: input_fd_vol_expansion_coefficient is currently not used
+        #       it might be used in future for solids 
         self.parameterWidget.label_category.setText(self.obj.Category)
         if self.obj.Category == "Fluid":
             self.parameterWidget.groupBox_mechanical.setVisible(0)