Doc how balance_value works

[skip ci]

app/serializers/api/admin/customer_with_balance_serializer.rb

@@ -2,6 +2,10 @@
 
 module Api
   module Admin
+    # This serializer relies on `object` to respond to `#balance_value`. That's done in
+    # `CustomersWithBalance` due to the fact that ActiveRecord maps the DB result set's columns to
+    # instance methods. This way, the `balance_value` alias on that class ends up being
+    # `object.balance_value` here.
     class CustomerWithBalanceSerializer < CustomerSerializer
       attributes :balance, :balance_status