As a starter for ten, I'd have thought that the user documentation should be task orientated. (i.e: How to achieve 'X'.)
In terms of the developer documentation, defining the meaning of any non-obvious fields in your schemas, how they're used and the relationships between different tables, etc. would be a good start. (I'm presuming your VBA code is well commented, etc.) You may also want to examine the existing "Documenting Visual Basic with Doxygen" question/answer.