The documentation sources are written in DocBook , which is a markup language defined in XML . In what follows, the terms DocBook and XML are both used, but technically they are not interchangeable.
DocBook allows an author to specify the structure and content of a technical document without worrying about presentation details. A document style defines how that content is rendered into one of several final forms. DocBook is maintained by the OASIS group . The official DocBook site has good introductory and reference documentation and a complete O'Reilly book for your online reading pleasure. The NewbieDoc Docbook Guide is very helpful for beginners. The FreeBSD Documentation Project also uses DocBook and has some good information, including a number of style guidelines that might be worth considering.