[SCSI] config: Make need for SCSI_CDROM clearer

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>