bootctl: separate boot loader specific commands in man and --help

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.