I would bet that any popular open-source project became popular before it had any decent documentation (assuming it has any now). The biggest block, for me, when writing documentation, is that I'm writing for a mystery audience, I don't know what their problems will be, or what questions they will ask. But once your project is used widely, documentation is just a matter of refactoring the mailing lists. And that's an itch some developers are willing to scratch - nobody wants to answer the same questions over and over.