[docs] xmldocs output formats

[Davor Ocelic]

> Good -- it will be nice to have a PDF version of our docs for
> printing. 

Sure, actually the thing is that DocBook uses one path to produce html
output, and another path (called "FO") for all other formats.

So when we add the generic/basic FO support, we get dvi/ps/pdf/txt/man
formats "for free". I'm especially looking forward to manpages for
the reference pages!