[VLM]

plantUML meets doxygen would be interesting.

I was involved in a very large system a decade ago that used robodoc. Robodoc has less features than doxygen but worked across more languages.

Swagger.io is a nice system although limited exclusively to REST APIs.

Anyway the examples above rendered extremely quickly even in 2010 and you could scaffold out entire ideas more or less in real time to get a whiteboard-ish experience assuming everyone on a conference call had a web browser window open.