But the barriers to contributing are high. You may not only need to learn about the application, but you need to learn any number of arcane editing and versioning tools, and then convince someone in authority to accept and include your changes.
I will appreciate the natural aversion to this idea, but we need *another project* to mediate the needs and products of open source documentation.
There are some translation websites that have been fabulously successful by making it extremely easy for volunteers to contribute translations to projects. There's nothing (that I know of) that makes it easy to match up the existing bad documentation, the user needs for new documentation, and the tools needed to easily integrate new documentation. There are some good documentation production tools - that doesn't need solving again - but the process needs some automation.
Wikis are better than nothing, but the structure is not guided and volunteers will always need guidance to succeed (and obviously wikis haven't solved the problem). The Arch Wiki is really great, but that's sadly an aberration, not typical. Nearly every Sourceforge wiki is useless, for instance. Arch has earned itself a kick-ass community and I bet a small number of volunteers were instrumental in "erecting the cathedral".
Most people blog on their own sites due to the obstacles involved with contributing good documentation. Fundamentally, it's a coordination problem, and that's something tools are good at solving.