Drools6 and jBPM6 are introducing a new public API called kie-api. It is very similar to the Drools5 and jBPM5 API in a lot of areas, but some areas were added or refactored as well. The API itself should have full Javadocs, and more documentation is currently under development as well.
For people using the old knowledge-api from Drools5 and jBPM5, this is still backwards compatible, so you should be able to use either