MAN: sssd-secrets documentation
    
    Resolves:
        https://fedorahosted.org/sssd/ticket/3053
    
    Documents the API and the purpose of the sssd-secrets responder.
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>