[models] update / doc

uncloud_pay/models.py

@@ -56,7 +56,7 @@ def default_payment_delay():
 # See https://docs.djangoproject.com/en/dev/ref/models/fields/#field-choices-enum-types
 class RecurringPeriod(models.IntegerChoices):
     """
-    We don't support months are years, because the vary in length.
+    We don't support months are years, because they vary in length.
     This is not only complicated, but also unfair to the user, as the user pays the same
     amount for different durations.
     """
@@ -648,6 +648,9 @@ class Product(UncloudModel):
 
     @property
     def one_time_price(self):
+        """
+        Default is 0 CHF
+        """
         return 0
 
     @property