[IA64] Kconfig: Add help text for IA64_SGI_SN2 config option.

Mark Goodwin suggested that we document our platform a bit.  This patch adds
help text for the sn2 specific kernel option.
Signed-off-by: Jesse Barnes <jbarnes@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/Kconfig

@@ -72,6 +72,12 @@ config IA64_HP_ZX1
 
 config IA64_SGI_SN2
 	bool "SGI-SN2"
+	help
+	  Selecting this option will optimize the kernel for use on sn2 based
+	  systems, but the resulting kernel binary will not run on other
+	  types of ia64 systems.  If you have an SGI Altix system, it's safe
+	  to select this option.  If in doubt, select ia64 generic support
+	  instead.
 
 config IA64_HP_SIM
 	bool "Ski-simulator"