improve generation of model comments
Add support of inheriting documentation from parent classes and interfaces. To inherit the documentation of a class or interface there are two choices: 1. Add no comment in the subclass / implementing class and add the Override annotation. 2. Add the {@inheritDoc} annoattion in the comment. In this case you can add an additional comment text wich will be used as implementation note. This also support using the comment of the @return tag if there is no first sentence provided.