Add a note in the docs about Fact type definition in DRL files

As discussed in this thread (https://kie.zulipchat.com/#narrow/stream/232676-kogito/topic/rules-codegen-error) on kie zulipchat, the DRL samples in the docs use an unsupported (at least at this moment) feature for declaring Fact Types in DRL files.

The correct way to get RuleUnits working in Kogito at the moment is to declare Fact Types and RuleUnit DataSources using Java Class files.