Also I made the mistake of writing the manual in MS Word. At the time, I couldn't find a standard way to insert pictures into a TeX document, or I would have used that. (Now, of course, there is a standard way.) There is an effort underway to convert the manual into a web-based format in a git repo that anyone can contribute to, but that effort is 90% done and you know what that means! :)
So it's getting better.
The other thing to remember is that the core Snap! development team is just two guys, Brian Harvey and Jens Mönig. When they're focused on things like trying to figure out how to get macros into Snap!, so that Snap! can truly be a Lisp (right now it's only most of a Lisp), they tend to leave the documentation effort to the community. If they had a larger team I might fault them for that, but with just two guys, I can't really blame them for focusing their efforts on things the community is less able to do, and leaving things the community can do up to the community.
Our team has officially grown to six people, adding Bernat Romagosa, Jadga Hügle, Michael Ball, and Joan i Pelegay. And several Snap! users have made major contributions, especially to libraries that extend the reach of the language. But the interpreter itself is still all Jens.