Update lv_obj_refresh_style description (#1746)

src/lv_core/lv_obj.c

@@ -1332,6 +1332,7 @@ bool lv_obj_remove_style_local_prop(lv_obj_t * obj, uint8_t part, lv_style_prope
 /**
  * Notify an object (and its children) about its style is modified
  * @param obj pointer to an object
+ * @param part the part of the object which style property should be refreshed.
  * @param prop `LV_STYLE_PROP_ALL` or an `LV_STYLE_...` property. It is used to optimize what needs to be refreshed.
  */
 void lv_obj_refresh_style(lv_obj_t * obj, uint8_t part, lv_style_property_t prop)