Default APR tier enrollment guidance clarified

Broker API

Clarifies how default APR tiers work for High Yield Cash enrollment. No fields or behavior change.

is_default on the APRTier schema now explains that each currency has at most one default tier and that an account signing the required Alpaca Customer Agreement revision without cash_interest may be enrolled in that tier automatically. Its example is now false.
The cash_interest request schema on Create an Account carries the same clarification.
Partners should not rely on default-tier enrollment and should always set apr_tier_name explicitly when creating or updating an account.