docs: Update nosmt support for arm64

docs: Update nosmt support for arm64

commit eed4583bcf9a6 ("arm64: Kconfig: Enable HOTPLUG_SMT") enable
HOTPLUG_SMT for SMT control for arm64, but the documentation was
not updated accordingly to reflect that ARM64 now supports control SMT
via boot parameter and sysfs knobs:

1. Boot parameters:

nosmt:          Disable SMT, can be enabled via sysfs knobs.
nosmt=force:    Disable SMT, cannot be enabled via sysfs knobs.

2. Runtime sysfs controls:

Write "on", "off", "forceoff" or the number of SMT threads (1, 2, ...)
to /sys/devices/system/cpu/smt/control.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260417032540.3720627-1-ruanjinjie@huawei.com>