As I've created a couple of these in my time, I think I have a pretty good take on what should be in a UX Architecture. Here's the highlights:
Foundational Research: How did you come to the design direction that you are documenting and following? What research did you leverage? This helps to support your design architecture and help "sell" it to anyone who may question your design direction.
Personas: Representative users who the design will help/support. The personas need to be based on research and interviews with actual users, and not be "imaginary friends." These personas represent who the design is for.
The Conceptual model/design: The vision/idea underlying the architecture. This can be a narrative, a visual representation, or both. It should be simple, easy to grasp, and obvious. It's the "elevator speech" that you will use to communicate the design.
Information Architecture: The domains and subdomains of the solution that is being designed. This should be, optimally, based on how users perceive the domains, and not your "guess" – use tools like Treejack to test your IA before documentation an execution.
OPTIONAL: Pattern library and process flows. Document how users accomplish the core tasks in the experience.