if there is any more Parser documenation floating out there it would be good to know of it.
As usual, the documentation falls far behind the code, and that is definitely the case for the Parser (not called MathObjects). There is some documentation in webwork2/doc/parser, but it is pretty sparse, and doesn't treat all the situations of interest. Many of the source files (particularly those in pg/macros) have some comments at the top that explain how to use them. I have also tried to post examples in the old discussion board (and will be in this new one) as people ask questions, partly in an attempt to provide at least some documentation, but there really needs to be much more. Mike has been working on some of that, and we hope to have more done this summer.
Your best bet may be to ask questions here, as I try to be pretty responsive to those, when I can.