Documentation: Improve softlockup_panic= description text

It should say what that <integer> range is and what that integer value
means. I had to look at the code...

Signed-off-by: Borislav Petkov <bp@suse.de>
[jc: changed non-null to nonzero]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -3885,6 +3885,12 @@
 			[KNL] Should the soft-lockup detector generate panics.
 			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.
+
 	softlockup_all_cpu_backtrace=
 			[KNL] Should the soft-lockup detector generate
 			backtraces on all cpus.