'Jersuite' is the name of a set of Vim plugins which I've hopefully written by the time you're reading this. Those plugins require some common infrastructure to interact with each other correctly, and that infrastructure is here in the vim-jersuite-core plugin.
Since vim-jersuite-core will only ever be used directly by other Vimscript developers (and I'd consider myself lucky if anyone but me ever uses it), its documentation doesn't extend beyond comments in the code and a general overview in doc/vim-jersuite-core.txt. There's no centralized API Reference.