I would also recommend using some sort of Wiki for "general" documentation (introduction to a module, giving an overview of the whole code library/project)

Where I work they have recently installed Trac, which is, to quote from the home page, "an enhanced wiki and issue tracking system for software development projects". Personally I find it quite handy, effective and useful both as a web-based interface to svn and as a wiki and I particularly appreciate its terse and clear, minimalistic, look & feel.