Tweak documentation to RB_ constants to reflect current use

RB_ASKNAME is no longer instructions to the boot loader to request a
prompt for which kernel to boot. Instead, it asks for what the root
file system to use. RB_INITNAME is unused, and never has been in
FreeBSD as far as I can tell. Remove it from the documentation and fix
comment. RB_SELFTEST and RB_MINIROOT likewise (though they were
completely undocumented). These last three constants can likely just
be deleted as nothing references them (even to set useless bits).

RB_ASKNAME doesn't actually survive reboot, however, so needs to be
communicated to the bootloader via other means. If the bootloader sets
it, though, it will be honored.