UPDATING: Add belated note about hints for old-school ISA devices

Back in July, jhb started a cleanup of our hints. We've removed several
that have been a problem for a long time (including some that had
workarounds for VM images). We've also moved uarts to ACPI only, so they
act more like wiring than enumeration hints. Add a note about this and
ways you can find if your system is affected. Old APU1 routers with
coreboot are affected, but newer APU2 devices are not. Others likely are
as well. The affected systems will be primarily very ancient systems
that pre-date ACPI (i586 arch pentium and older typically), as well as
some older coreboot and similar non-conventional boot firmware setups
(primarily in the embedded space). FreeBSD 15 will require ACPI or
PNPBIOS enumeration for everything by default, so any non-enumerated
devices via these means will require hints. Add notes on how to grep
for affected systems before upgrading.

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