formatcaps: Rework and add stubs to document
    
    At the moment we are missing even basic documentation on our
    capabilities XML. Without demand on completeness, I'm
    reorganizing the document structure and adding very basic
    documentation to two major components of the capabilities XML.
    These stubs are intended to be enhanced in the future.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>