bootctl: separate boot loader specific commands in man and --help
    
    bootctl implements three types of operation: those that work with an EFI
    boot loader, those which work with any EFI boot loader that implements
    the boot loader spec + interface, and finally those specific to sd-boot.
    Previously the --help text and the man page mixed them all up. Let's put
    them clearly in three separate sections however, to communicate clearly
    what is supposed to work everywhere, and what is specific to
    systemd-boot or boot loaders implementing the two specs.
    
    This adjusts wording here and there, but is mostly just about
    re-ordering existing docs, and putting them under new sections.