The documentation sources are written in
DocBook, which is a markup language
superficially similar to HTML. Both of these
languages are applications of the Standard Generalized
Markup Language, SGML, which is
essentially a language for describing other languages. In what
follows, the terms DocBook and SGML 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.