Add clarifying code comment

Author: RafaelKayumov

Modules/Sources/Yosemite/Model/Booking/Booking+ReadOnlyConvertible.swift

@@ -13,8 +13,13 @@ extension Storage.Booking: ReadOnlyConvertible {
         allDay = booking.allDay
         cost = booking.cost
         customerID = booking.customerID
+
+        /// Falling to back to existing values in case if new values are absent
+        /// Booking returned when sending a `PUT` request to `bookings/{booking_id}`
+        /// doesn't contain `date_created` and `date_modified` values.
         dateCreated = booking.dateCreated ?? dateCreated
         dateModified = booking.dateModified ?? dateModified
+
         endDate = booking.endDate
         googleCalendarEventID = booking.googleCalendarEventID
         orderID = booking.orderID