explain the hidden scsi_wait_scan Kconfig variable

People keep sending patches to expose CONFIG_SCSI_WAIT_SCAN as a tunable
item.  These patches aren't accepted upstream, so let's stop the ongoing
irritation of people due to the unconditionally installed module and its
Kconfig symbol.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/Kconfig

@@ -258,10 +258,21 @@ config SCSI_SCAN_ASYNC
 	  or async on the kernel's command line.
 
 config SCSI_WAIT_SCAN
-	tristate
+	tristate  # No prompt here, this is an invisible symbol.
 	default m
 	depends on SCSI
 	depends on MODULES
+# scsi_wait_scan is a loadable module which waits until all the async scans are
+# complete.  The idea is to use it in initrd/ initramfs scripts.  You modprobe
+# it after all the modprobes of the root SCSI drivers and it will wait until
+# they have all finished scanning their buses before allowing the boot to
+# proceed.  (This method is not applicable if targets boot independently in
+# parallel with the initiator, or with transports with non-deterministic target
+# discovery schemes, or if a transport driver does not support scsi_wait_scan.)
+#
+# This symbol is not exposed as a prompt because little is to be gained by
+# disabling it, whereas people who accidentally switch it off may wonder why
+# their mkinitrd gets into trouble.
 
 menu "SCSI Transports"
 	depends on SCSI