qapi/machine.json: Remove zero value reference from SMPConfiguration documentation

qapi/machine.json: Remove zero value reference from SMPConfiguration documentation

Commit 1e63fe685804 ("machine: pass QAPI struct to mc->smp_parse")
introduced documentation stating that a zero input value for an SMP
parameter indicates that its value should be automatically configured.
This is indeed how things work today, but we'd like to change that.
Avoid documenting behaviors we want to leave undefined for the time
being, giving us freedom to change it later.

Fixes: 1e63fe685804 ("machine: pass QAPI struct to mc->smp_parse")
Signed-off-by: Andrew Jones <drjones@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>