There is no documentation at this time.
The file ayaparser.aya is a correct source file for generating the ayaparser used by the aya program. It uses almost all the "functionalities" of ayauhteotl, so it is a good example, but a rough one for a starting point.
If you'r interrested in understanding the syntax/semantic of ayauhteotl, and have time to write some doc to go here, I can help you by mail