boot_*.8: reference installboot.8, rectify FFS note on sparc64

boot_*.8: reference installboot.8, rectify FFS note on sparc64

On sparc64, installboot(8) does not deal with any file system; its -p is a
NOOP and according to CVS history the old sys/arch/sparc64/installboot/
never prepared a file system, either.

Remove this single misleading mention alltogether and rely on the now
consistently cross-linked MI installboot(8) for details, if any.

This syncs the list of currently existing MD boot_*(8/*) pages with the list
of usr.sbin/installboot/*_installboot.c files.

boot_alpha(8/alpha) already references installboot(8) but still uses
sys/arch/alpha/stand/installboot/ instead of usr.sbin/installboot/.

boot_luna88k(8/luna88k) exists but luna88k does not use installboot at all.

installboot(8) has additional support for efi (armv7, arm64, riscv64),
landisk, loongson, octeon and powerpc64 but those architectures currently
lack their own MD boot_*(8/*) manual.

OK miod