Documentation: document hung_task_panic kernel parameter

This parameter has been around since commit e162b39a ("softlockup:
decouple hung tasks check from softlockup detection") in 2009 but was
never documented.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -1341,12 +1341,21 @@
 			x86-64 are 2M (when the CPU supports "pse") and 1G
 			(when the CPU supports the "pdpe1gb" cpuinfo flag).
 
+	hung_task_panic=
+			[KNL] Should the hung task detector generate panics.
+			Format: <integer>
+
+			A nonzero value instructs the kernel to panic when a
+			hung task is detected. The default value is controlled
+			by the CONFIG_BOOTPARAM_HUNG_TASK_PANIC build-time
+			option. The value selected by this boot parameter can
+			be changed later by the kernel.hung_task_panic sysctl.
+
 	hvc_iucv=	[S390]	Number of z/VM IUCV hypervisor console (HVC)
 				terminal devices. Valid values: 0..8
 	hvc_iucv_allow=	[S390]	Comma-separated list of z/VM user IDs.
 				If specified, z/VM IUCV HVC accepts connections
 				from listed z/VM user IDs only.
-
 	keep_bootcon	[KNL]
 			Do not unregister boot console at start. This is only
 			useful for debugging when something happens in the window