Documentation: power/cpuidle: Document the CPU system wakeup latency QoS

Documentation: power/cpuidle: Document the CPU system wakeup latency QoS

Let's document how the new CPU system wakeup latency QoS limit can be used
from user space, along with how the constraint is taken into account for
s2idle and cpuidle.

Reviewed-by: Dhruva Gole <d-gole@ti.com>
Reviewed-by: Kevin Hilman (TI) <khilman@baylibre.com>
Tested-by: Kevin Hilman (TI) <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/20251125112650.329269-7-ulf.hansson@linaro.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>