Documentation: Better document the softlockup_panic sysctl

Commit 9c44bc03ff ("softlockup: allow panic on lockup") added the
softlockup_panic sysctl, but didn't add information about it to the file
Documentation/admin-guide/sysctl/kernel.rst (which in that time certainly
wasn't rst and had other name!).

This patch just adds the respective documentation and references it from
the corresponding entry in Documentation/admin-guide/kernel-parameters.txt.

This patch was strongly based on Scott Wood's commit d22881dc13
("Documentation: Better document the hardlockup_panic sysctl").

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Link: https://lore.kernel.org/r/20200310183649.23163-1-gpiccoli@canonical.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -4516,10 +4516,10 @@
 			Format: <integer>
 
 			A nonzero value instructs the soft-lockup detector
-			to panic the machine when a soft-lockup occurs. This
-			is also controlled by CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC
-			which is the respective build-time switch to that
-			functionality.
+			to panic the machine when a soft-lockup occurs. It is
+			also controlled by the kernel.softlockup_panic sysctl
+			and CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC, which is the
+			respective build-time switch to that functionality.
 
 	softlockup_all_cpu_backtrace=
 			[KNL] Should the soft-lockup detector generate

Documentation/admin-guide/sysctl/kernel.rst

@@ -1036,6 +1036,20 @@ NMI.
 = ============================================
 
 
+softlockup_panic
+=================
+
+This parameter can be used to control whether the kernel panics
+when a soft lockup is detected.
+
+= ============================================
+0 Don't panic on soft lockup.
+1 Panic on soft lockup.
+= ============================================
+
+This can also be set using the softlockup_panic kernel parameter.
+
+
 soft_watchdog
 =============