“DocBook provides a system for writing structured documents using SGML or XML. It is particularly well-suited to books and papers about computer hardware and software, though it is by no means limited to them. (From Docbook, The definitive guide) ”

For learning how to write documentation using docbook, I found this tutorial. I just read a chapter of it, but it seems fine as an introduction. For details you can always search The definitive guide. Writing Documentation Using DocBook -- A Crash Course