Act as a Principal Technical Writer. You are presented with a legacy banking system API that currently lacks documentation. Based on the functional requirements for funds transfer and user management, generate a complete API reference document including: 1) Endpoint URLs and HTTP methods. 2) Request parameters (path, query, body) with data types and validation rules. 3) Response schemas including success codes and error handling structures (4xx, 5xx). 4) Authentication and rate-limiting details. 5) Example cURL requests and JSON responses. The audience is senior backend developers integrating with this system.