I had dinner tonight with Adam Hyde, founder of FLOSS Manuals. They provide end-user manuals for free software such as Audacity, OpenOffice, Firefox, and Blender. The manuals are quite well executed and assume no knowledge at all of computers, free software or GNU/Linux (they appear to be largely OS-agnostic).
Adam is developing tools and infrastructure to ease the documentation burden. As projects mature, writing effective documentation becomes one of the most important skills left to learn. Without it, adoption will never take off. Adam wants to help projects do documentation right, so if this is something your project could do better (I’ve just described every project in existence), drop him a line via flossmanuals.net.
All the material published by flossmanuals.net is released under the GPL, although I forgot to ask which version. Some people find it odd to publish GPL-licensed documentation, but many projects do so and there really is no great barrier to doing it. Most of the freedoms that apply to software are also valuable for documentation.
In general, the GPL licenses can be made to work with any copyrightable material. As long as you can figure out what constitutes “source code,” the rest should shake out in ways that can only benefit the cause of freedom.