Document some last-minute failure modes as show-stopper for 5.2R:

• ACPI panics during shutdown on SMP systems due to failure to unregister an eventhandler (njl has a patch).
• random_harvest panic due to last minute entropy harvesting changes.

Observe that remaining turnstile panics may well be secondary panics,
and that those reporting turnstile panics should be sure to include
a full stack trace.