This is the most recent notation reference that I can find, but it's still for version 1.5..

www.omg.org/docs/formal/03-03-10.pdf

It briefly describes constraints in general, but doesn't really get into how they are applied --such as for Activity diagrams.

adrian wrote I don't believe that the details of modeling an activity diagram have changed significantly in UML 2.0 but then I have not read in full the 2.0 specification. Here's the official UML 2.0 specification (a bit hard to read): http://www.omg.org/spec/UML/2.1.2/  -> take a look at the Superstructure For the official UML info, your main resource is http://www.uml.org;  there you'll find links to many UML 2.0 resources including articles, tutorials, tools,  etc. - Adrian

Thanks for assisting me, Adrian.

I reviewed the Superstructure document earlier today; it's obviously not something that I can just peruse and expect to comprehend.  I just find it hard to accept that there is no simple reference about OR constraints and how they can be applied toward Activity diagrams.  I was about to start using forks for what I thought was a perfectly legitimate reason, and then I found one obscure example that used a decision instead of a fork which happens to be, according to you, the correct way to model that scenario.  That's pretty frustrating for a beginner! 

Again, thanks for the help.