ARM: 7816/1: CONFIG_KUSER_HELPERS: fix help text

Commit f6f91b0d ("ARM: allow kuser helpers to be removed from the
vector page") introduced some help text for the CONFIG_KUSER_HELPERS
option which is rather contradictory.

Let's fix that, and improve it a little.

Cc: <stable@vger.kernel.org>
Signed-off-by: NNicolas Pitre <nico@linaro.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/Kconfig

@@ -809,15 +809,18 @@ config KUSER_HELPERS
 	  the CPU type fitted to the system.  This permits binaries to be
 	  run on ARMv4 through to ARMv7 without modification.
 
+	  See Documentation/arm/kernel_user_helpers.txt for details.
+
 	  However, the fixed address nature of these helpers can be used
 	  by ROP (return orientated programming) authors when creating
 	  exploits.
 
 	  If all of the binaries and libraries which run on your platform
 	  are built specifically for your platform, and make no use of
-	  these helpers, then you can turn this option off.  However,
-	  when such an binary or library is run, it will receive a SIGILL
-	  signal, which will terminate the program.
+	  these helpers, then you can turn this option off to hinder
+	  such exploits. However, in that case, if a binary or library
+	  relying on those helpers is run, it will receive a SIGILL signal,
+	  which will terminate the program.
 
 	  Say N here only if you are absolutely certain that you do not
 	  need these helpers; otherwise, the safe option is to say Y.