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

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Signed-off-by: Bernhard Walle <bwalle@suse.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>