Add warning notes for known quirkly ata chipsets

[Dieter]

> I don't know any others offhand

Word is that nforce4 plus Maxtor or Hitachi disks gives
data corruption.  Nforce4 plus Seagate is said to be okay.

http://forums.nvidia.com/lofiversion/index.php?t8171.html

I have nforce4-ultra with 4 Seagate 7200.8 SATA drives and have
not observed any data corruption.  I have have copied many
multi-GB files from one disk to another, and afterwards cmp(1)
says they are identical.

I picked the nforce4-ultra because the other AMD64 chipsets
did not have NCQ.  I have not had any problems with the nf4
chipset. (But then I don't have any Maxtor or Hitachi disks
hooked to it.)

On the other hand, I *have* seen data problems with a Maxtor
PATA drive on NetBSD/alpha (not a nforce4 chipset, obviously).
Read the same sector three times and get three different results.