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Mention ATAPI. We could insert an essay about libata and ide-scsi etc but
the failure case is someone enables it which is just fine so keep it
simple.

(Revised text from suggestion by Matthew Wilcox)

Closes #7736
Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>