I can not say enough good things about Asciidoc. It has a very simple markup syntax, can generate everything from pdf to roff, portable to implement and very easily inserted into any wiki with only a few minor changes.
Even in its markup state, its very, very easy to read. The only thing I have to fiddle with when using it are tables, but that's not too difficult.
If you keep the text formatted files in your repository, revision tracking is quite simple.
For code documentation, I use doxygen.
