The technical documentation of the REST-API is contained in the search application itself and can be accessed via [your NG server]/fact-finder/swagger-ui.html . It can be also accessed via the following URL:
Swagger Link
The interface offers not only the parameter documentation, but also the option to test a query and review the result. The documentation was created with Swagger, you can use Swagger Codegen to generate a client for it and use it for your code.
ASN information is contained in the search query, so you have to use a search - method. The returned data can be found in the asnGroups -block.