Many of the variables are common in CMake, e.g. *_INCLUDE_DIRS, *_LIBRARIES, etc. There is no ament specific documentation for those.
Other variables are commonly not being used by users and are therefore not mentioned anywhere.
In general there is not much documentation beyond the API docs (this is ticketed in ament/ament_cmake#2). If there is a specific variable you would like to know more about please don’t hesitate to ask. I am happy to provide feedback.