I am not sure about the state of the FrameMaker ExtendScript documentation, but you can download one of the FrameMaker FDKs (Frame Developers Kit) and use its documentation.
The Extendscript documentation is atrocious. All they did is create a constants chapter that merely lists all the constants for all methods but does NOT define what the constants do NOR tell you what method or methods the constants are for. Its absolutely maddening.
That said you are right about the FDK documentation. This actually tells you what the parameters for the open method are and defines them.