[SCSI] Update wording of CONFIG_SCSI_MULTI_LUN help

I had to set CONFIG_SCSI_MULTI_LUN to y in order to get my SE W595
working when plugging it as a mass storage. Looking at SCSI option to
get a phone behaving correctly was convoluted to say the least. There
are quite a few other reports about USB card readers needing this option
as well. This patch improves the help text to make the use of the option
more obvious.

Signed-off-by: Eric Piel <eric.piel@tremplin-utc.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>