grubby-bls: update man page to match options in current wrapper script
    
    The man page of the old grubby tool is still shipped in the grubby package
    but instead there should be a man page that only documents the options for
    the wrapper grubby script that is used to manage the BLS config snippets.
    
    Current man page is confusing for users so let's add one that contains the
    options that are supported by the wrapper script but no unsupported ones.
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>