docs: document panic fully in sysctl/kernel.rst

The description of panic doesn’t cover all the supported scenarios;
this patch fixes that, describing the three possibilities (no reboot,
immediate reboot, reboot after a delay).

Based on the implementation in kernel/panic.c.
Signed-off-by: Stephen Kitt <steve@sk2.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/sysctl/kernel.rst

@@ -559,9 +559,15 @@ The default is 65534.
 panic
 =====
 
-The value in this file represents the number of seconds the kernel
-waits before rebooting on a panic. When you use the software watchdog,
-the recommended setting is 60.
+The value in this file determines the behaviour of the kernel on a
+panic:
+
+* if zero, the kernel will loop forever;
+* if negative, the kernel will reboot immediately;
+* if positive, the kernel will reboot after the corresponding number
+  of seconds.
+
+When you use the software watchdog, the recommended setting is 60.
 
 
 panic_on_io_nmi