First, thank you Dave, it’s a great job. I love it.
Now my two cents:
I bet for the three categories you propose. The only change I would do is moving the documentation related to people who are in the administration groups in Tryton to users category. Maybe noting that kind of documentation to the admin user.
The last change I would IMHO is related to the Project: Client:
As far as I understand the aim for both clients is to give the same behavior, look and feel to the Tryton user, so I wouldn’t build one documentation for each one else only one noting the differences that they have when be required.