[doc] begin describe replacing orders

doc/uncloud-manual-2020-08-01.org

@@ -41,6 +41,15 @@
    Order register the one time price and the recurring price. These
    fields should be treated as immutable. If they need to be modified,
    a new order that replaces the current order should be created.
+*** Replacing orders
+    If an order is updated, a new order is created and points to the
+    old order. The old order stops one second before the new order
+    starts.
+
+** Product and Product Children
+   - A product describes something a user can buy
+   - A product inherits from the uncloud_pay.models.Product model to
+     get basic attributes
 * Products
 ** VPN
 *** How to add a new VPN Host