The documentation generally does apply correctly between OS-9 and NitrOS-9.
To understand the advantages over OS-9, it is good to look back that the different eras of the project.
In the beginning, Bill Nobel, Curtis Boyle, and Wes Gale decided to rewrite parts of the operating system to support to newly discovered features of the 6309 microprocessor. OS-9 is very modular and this kind of updating is possible even without the original source code.
Then Alan Dekok added to the project to use better, smaller algorithms in other parts of the OS (speciffically: GrfDrv).
Later still, Boisy Pitre bought the project online with source code control and added more features.