What are best practices for documenting XSLT?
I've tried to do it using UML, but it seems like putting a square peg in a
round hole - not a real good fit (I think because UML, at least in my mind, is
for imperative code and perhaps even more specifically for OOP).

Have you ever been tasked to describe _graphically_ what a transform is doing
and, if so, how have you done it?