What I prefer is a relatively small written document to give an overview of the application/system, common usage scenarios, and big caveats or tricky spots. Ed

What I prefer is a relatively small written document to give an overview of the application/system, common usage scenarios, and big caveats or tricky spots. Edge cases and smaller design decisions on the other hand should go into a kind of wiki and/or bug database that is searchable based on entity(s), issue/feature type, programmer, commentator (content author), change requester, etc. There will often be back-and-forth discussions that would bloat up a formal document. But to be useful, it requires a good knowledge-base, both product-wise and management-wise.