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>