When EFA is installed, REST API documentation is available as an HTML reference: http://<host_ip>/docs.
The REST API is specified by OpenAPI and Swagger.
Specific API guides for the EFA services are available on the Extreme Networks website. Select Extreme Fabric Automation here: https://www.extremenetworks.com/support/documentation/product-type/software/. And then select the version of EFA you want to work with.