Does the online API reference refer to the tip or to the latest "release" build? I've found some things in there that don't seem to be in my source tree. Also, in general, how does one differentiate between different versions of JUCE and their accompanying documentation?
It's the latest tip, not the last release.
At the moment I run a script to update it, which I do fairly regularly, but we'll soon be automating this so that it gets regenerated automatically whenever we push to the master branch.
That's great, but I'm using the tagged "3.1.1" version in my project. Where can I find a version of the documentation that matches the release version of your product?
The API docs are held on github in a repo - you can roll it back to any point you want to.