frontend, doc: put cost=1100 to multilib repo
    
    I noticed that we miss the cost= argument in multilib repofile when
    documenting the recently added multilib option.  The should help users
    so they don't install the packages from 32/64bit repositories by random.
    
    The docs originally mentioned cost 900/1000, but since the default is
    1000 - the main repo should stay default (as with non-multilib
    repofiles) and we should special case the multilib repository.  That's
    why it has higher cost 1100 for now.