A few months before, I wrote a blog entry about architecture diagrams. I mentioned that UML can be used for describing architectural concepts, but I didn't write too much about that. Perhaps it was a mistake, because you might think, that UML is a good choice for these kind of tasks. Bu it's not.
To describe an architecture, we need a way to describe entire systems, not just components and connections. UML has deployment diagram, of course, but it lacks well-defined semantic meaning behind diagram elements. It means, that a deployment diagram is not an unambiguous description of a system, since different people assign different meaning to the elements of a diagram.
There are some more formal approaches, like FMC (Fundamental Modeling Concepts). Its concepts are nice and well-founded, but only a few people use it, unfortunately.