Too much documentation has never been an issue in projects I've worked on in the past. I feel there's enough when, at a glance, new people coming into the project can be caught up quickly. The way I write my stories allows QA to build an easily readable Cucumber test suite that serves as documentation on features, and I let developers create their own documentation on project set up. Other documentation may include mockups and flow charts but are primarily used for estimation and feature building.