Documentation: Better document the hardlockup_panic sysctl

Commit ac1f591249 ("kernel/watchdog.c: add sysctl knob
hardlockup_panic") added the hardlockup_panic sysctl, but did not add it
to Documentation/sysctl/kernel.txt.  Add this, and reference it from the
corresponding entry in Documentation/admin-guide/kernel-parameters.txt.

Signed-off-by: Scott Wood <swood@redhat.com>
Acked-by: Christoph von Recklinghausen <crecklin@redhat.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -2538,6 +2538,9 @@
 			This is useful when you use a panic=... timeout and
 			need the box quickly up again.
 
+			These settings can be accessed at runtime via
+			the nmi_watchdog and hardlockup_panic sysctls.
+
 	netpoll.carrier_timeout=
 			[NET] Specifies amount of time (in seconds) that
 			netpoll should wait for a carrier. By default netpoll

Documentation/sysctl/kernel.txt

@@ -34,6 +34,7 @@ show up in /proc/sys/kernel:
 - hostname
 - hotplug
 - hardlockup_all_cpu_backtrace
+- hardlockup_panic
 - hung_task_panic
 - hung_task_check_count
 - hung_task_timeout_secs
@@ -313,6 +314,19 @@ will be initiated.
 1: on detection capture more debug information.
 ==============================================================
 
+hardlockup_panic:
+
+This parameter can be used to control whether the kernel panics
+when a hard lockup is detected.
+
+   0 - don't panic on hard lockup
+   1 - panic on hard lockup
+
+See Documentation/lockup-watchdogs.txt for more information.  This can
+also be set using the nmi_watchdog kernel parameter.
+
+==============================================================
+
 hotplug:
 
 Path for the hotplug policy agent.