1) For embedded systems, it makes no sense to include documentation
   of this kind on each image.  So we might as well not even build
   it.
2) For people bootstrapping systems from source code, documentation
   is the source of many cyclical build loops.  Allowing it to
   be disabled helps cut these loops.
3) The Docbook stylesheets are distributed as Zip files, and I refuse
   to put that bullshit into my build system.