Hi there 
You are right, this could be clearer. I will try to get this documentation updated.
You are right that you don’t have to set a capped amount when using RecurringApplicationCharge. You can charge a set rate each month, and not have a usage component.
This sentence in the REST documentation touches on the fact that both capped amount and terms are required properties when you want your recurring application charge to also include usage fees.
You need to include the capped_amount and terms properties in the body of your request when you create a recurring application charge with a price of $0.00.
You can also check the GraphQL documentation for creating usage billing plans, which does show explicitly that both fields are required.
Sorry about the confusion this has caused you, and I will look to get that documentation updated.