Clarify package description
    
    The grubby script was meant to be a temporary solution to provide backward
    compatibility with existing grubby users. But it turned out that grubby is
    the recommended approach to modify the boot loader entries.
    
    Let's make clear in the package description that this is not only meant to
    be used for legacy compatibility purposes, which may discourage its usage.
    
    Resolves: rhbz#1913299
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>