That is, or should be, a different set of docs. Design documents make lousy user documentation.

I think it depends on the sort of module. Ideally, in my view, the user documentation should be at least a part of the design documentation. At very least it represents the contract as offered. If that is hard to document, you have design problems.

That doesn't mean that's all of the documentation, but my experience is that clear user documentation is a general indication of clear design (exception made for Microsoft Xenix).