Finally, after considerable thought, I have chosen, not without the help of others, to use TeX for writing documentation. After two days of using it, I must admit that I enjoy it really much. There are few helpful online resource like TeX User Group, UK List of TeX FAQ, LaTex and BibTeX course notes (with lots of examples). For those who can read and understand Russian, TeX FAQ and TeX section of opensource.ru might be of some help.