[SCSI] scsi: Error handler description document

This patch adds Documentation/scsi/scs_eh.txt.  I've chosen plain
text over DocBook as most other scsi docs are in plain text and it's
more accessible.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>