Document panic_on_unrecovered_nmi sysctl

This adds "panic_on_unrecovered_nmi" sysctl to
Documentation/filesystems/proc.txt. The text is mainly taken from
http://readlist.com/lists/vger.kernel.org/linux-kernel/43/217998.html.
Signed-off-by: NBernhard Walle <bwalle@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/proc.txt

@@ -1321,6 +1321,18 @@ debugging information is displayed on console.
 NMI switch that most IA32 servers have fires unknown NMI up, for example.
 If a system hangs up, try pressing the NMI switch.
 
+panic_on_unrecovered_nmi
+------------------------
+
+The default Linux behaviour on an NMI of either memory or unknown is to continue
+operation. For many environments such as scientific computing it is preferable
+that the box is taken out and the error dealt with than an uncorrected
+parity/ECC error get propogated.
+
+A small number of systems do generate NMI's for bizarre random reasons such as
+power management so the default is off. That sysctl works like the existing
+panic controls already in that directory.
+
 nmi_watchdog
 ------------