And that's the description

classmethod GenerateMappingFromXdata(class As %String, ByRef mapping) as %Status

Get the JSON adaptor configuration information from class and property parameters.

BUT:
https://docs.intersystems.com/iris20212/csp/documatic/%25CSP.Documatic.cls?&LIBRARY=%25SYS&CLASSNAME=%25JSON.Generator

This class is used internally by InterSystems IRIS. You should not make direct use of it within your applications. There is no guarantee made about either the behavior or future operation of this class.