I am in the process of documenting a system I added to an open source project. Its not easy to write, particularly if one doesn't have a background in technical writing. The basic process the same though, you have to write for your audience, and revise.... a lot. In my case, I have been sending drafts of my work to other developers in the project to not only proofread, but to actually read through the directions and perform the listed tasks as an end-user would. So far the feedback has resulted in changes in both the documentation and the program to make things easier/clearer for the user.
I guess what I'm trying to say is, don't skip review and just post the documentation. Give the documentation and software to someone not familiar with it and see how they interpret and understand it. Listen to their feedback. Way too many developers don't (I'm looking at your Google!). Wikis are supposed to address this, but don't seem to engage enough people to actually contribute.