Documentation / CPU hotplug: Rephrase the outdated description for MADT entries

More than 256 entries in ACPI MADT is supported from ACPI 3.0, so the
information in should be Documentation/cpu-hotplug.txt updated.

[rjw: Changelog]
Suggested-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/cpu-hotplug.txt

@@ -370,8 +370,10 @@ A: There is no clear spec defined way from ACPI that can give us that
    CPUs in MADT as hotpluggable CPUS.  In the case there are no disabled CPUS
    we assume 1/2 the number of CPUs currently present can be hotplugged.
 
-   Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
-   in MADT is only 8 bits.
+   Caveat: ACPI MADT can only provide 256 entries in systems with only ACPI 2.0c
+   or earlier ACPI version supported, because the apicid field in MADT is only
+   8 bits. From ACPI 3.0, this limitation was removed since the apicid field
+   was extended to 32 bits with x2APIC introduced.
 
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