What I have is:

* Two different classes that are in different threads (say classA and classB).
* They do not know anything about each other (no includes / forward decls).
* When classA has finished something it posts a message (the routing is handled elsewhere), and classB receives it.

That's all really. In my class diagram, I want to show that classA and classB have messages passing between them (i.e. can communicate) and that they are in different threads.

Is this possible in UML class diagram? - is there a "communicates" relationship syntax?
If UML does not do this then I will just make up my own method to show this, but if there is a standard I may as well use it!