docs: Document possible_cpus parameter

The number of possible CPUs is set be kernel in early boot time through
some discovery mechanisms, like ACPI in x86. We have a parameter both
in x86 and S390 to override that - there are some cases of BIOSes exposing
more possible CPUs than the available ones, so this parameter is a good
testing mechanism, but for some reason wasn't mentioned so far in the
kernel parameters guide - let's fix that.

Cc: Changwoo Min <changwoo@igalia.com>
Signed-off-by: "Guilherme G. Piccoli" <gpiccoli@igalia.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240203152208.1461293-1-gpiccoli@igalia.com

Documentation/admin-guide/kernel-parameters.txt

@@ -4644,6 +4644,11 @@
 			may be specified.
 			Format: <port>,<port>....
 
+	possible_cpus=  [SMP,S390,X86]
+			Format: <unsigned int>
+			Set the number of possible CPUs, overriding the
+			regular discovery mechanisms (such as ACPI/FW, etc).
+
 	powersave=off	[PPC] This option disables power saving features.
 			It specifically disables cpuidle and sets the
 			platform machine description specific power_save