I stumbled upon a particular question on Stack Overflow that is not programming related, but it is development related. More architect work though.
Where should these kind of questions be migrated to have at least some chance of getting answered?
Generally speaking questions that are about anything in the SDLC other than implementation are on topic on Programmers. That includes software architecture, analysis & design, testing, etc, check out our FAQ for more details.
However the question you linked to isn't really suitable for us, I've read it twice and I'm not sure what the OP wants, or why their current diagram is insufficient (well I can see why it's insufficient, it's a mess, but that's my problem with it not theirs). I've posted a comment to the question pointing them to sequence diagrams, but that's more of a guess really.