[MTD] mtdoops: Document usage in Kconfig

Add usage instructions to Kconfig for mtdoops driver.
Signed-off-by: NPeter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>

drivers/mtd/Kconfig

@@ -286,6 +286,9 @@ config MTD_OOPS
 	  buffer in a flash partition where it can be read back at some
 	  later point.
 
+	  To use, add console=ttyMTDx to the kernel command line,
+	  where x is the MTD device number to use.
+
 source "drivers/mtd/chips/Kconfig"
 
 source "drivers/mtd/maps/Kconfig"