Improve JavaDoc on frontend-stubs

[blaumeiser-at-bosch]

Summary of the Improvement

Improve the javadoc situation of explicit apis explicitely in frontend-stubs component

Acceptance Criteria

• [ ] Javadoc for every 'official' api is documented

Definition of Done

• [ ] Acceptance criteria fulfilled
• [ ] The ticket describes what has been done and describes how the results can be verified (e.g. new Jenkins step is inserted, the report can be viewed at X)
• [ ] All known limitations are listed and it is described how they will be addressed (e.g., a ticket for the remaining stuff)
• [ ] Documentation is updated
• [ ] The result has been reviewed: Everything works as described and can be verified reading only the documentation