ARM: 9212/1: domain: Modify Kconfig help text

After the removal of set_fs() the reference to set_fs() is stale.
Alter the helptext to reflect what the config option really does.

Fixes: 8ac6f5d7 ("ARM: 9113/1: uaccess: remove set_fs() implementation")
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

arch/arm/mm/Kconfig

@@ -631,7 +631,11 @@ config CPU_USE_DOMAINS
 	bool
 	help
 	  This option enables or disables the use of domain switching
-	  via the set_fs() function.
+	  using the DACR (domain access control register) to protect memory
+	  domains from each other. In Linux we use three domains: kernel, user
+	  and IO. The domains are used to protect userspace from kernelspace
+	  and to handle IO-space as a special type of memory by assigning
+	  manager or client roles to running code (such as a process).
 
 config CPU_V7M_NUM_IRQ
 	int "Number of external interrupts connected to the NVIC"