Document behavior of compat when creating qcow2 volumes
    
    Commit bab2eda changed the behavior for missing compat attribute,
    but failed to update the documentation.
    
    Before, the option was omitted from qemu-img command line and the
    qemu-img default was used. Now we always specify the compat value
    and the default is 0.10.
    
    Reported by Christophe Fergeau
    https://bugzilla.gnome.org/show_bug.cgi?id=746660#c4