Add extensive JavaDoc documentation

[knittl]

Extend JavaDoc of annotations with snippets from the "big blue book" from Eric Evans or from the "DDD reference book" to make the concepts more easily understandable.
IMPORTANT: before starting on this issue, clarify any copyright related questions. Can we quote from the original big blue book by Eric Evans? Can we use text from the DDD reference? If not, what alternatives do we have?

[knittl]

E-Mail sent, waiting for reply to work on next steps.