Clearer descriptions and examples across Broker API schemas

Broker API

Improves how several Broker API schemas are documented and rendered. Values, required fields, and response shapes are unchanged.

Adds a request body description to the create-agreement endpoint.
Adds an example to the idempotency key conflict response on the create-journal endpoint (code: 42210000, message: idempotency key reused with a different request body).
account_type, account_sub_type, and minor_identity on AccountCreationRequest now render their own descriptions and the trading default for account_type correctly.
On AdminActionEventGeneral, the belongs_to and created_by descriptions no longer repeat the deprecation notice, which now renders on its own with its sunset date.