In design pharse, there are some objects that are specifically describe the system. Personally, I think they are quite fit to Struts.
In UML notation, there are 3 objects: Control, Entity and Interface.
In this sense, you may think DispatchAction class as a Control object, those views (JSPs or TransferObjects or even Form) are Entity objects, and the method callers are the Interface objects.
Thus, even your system make use of Struts, the UML can model them. Depends on the needs of your doucmentation, you pick the diagrams that you need to elaborate. The most common diagrams, I think, should be class, sequence and state diagrams.
You work with component and package and deployment diagrams when your system is ready to launch.