Source: Postman Learning Center
API (application programming interface) documentation is a reference outlining instructions on how to properly integrate and use APIs in your application.
It’s an all-inclusive manual containing exactly the information you need to use the API in your application(s). The documentation includes information about the API’s functions, return types, classes, arguments, and much more.
API documentation is traditionally done using text editors through industry-accepted API description formats, such as OpenAPI.