PM: runtime: Document that force_suspend() is incompatible with SMART_SUSPEND

pm_runtime_force_suspend() cannot be used with DPM_FLAG_SMART_SUSPEND, so
note this in the kerneldoc.

If DPM_FLAG_SMART_SUSPEND is set and the PM core cannot skip system resume
it will call pm_runtime_active() on the driver. This can lead to an
inconsistent state where:

  pm_runtime_force_suspend() called ->runtime_suspend

but

  device_resume_noirq() called pm_runtime_set_active()

This leaves the driver actually suspended but marked as active.
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/runtime.c

@@ -1864,6 +1864,10 @@ static bool pm_runtime_need_not_resume(struct device *dev)
  * sure the device is put into low power state and it should only be used during
  * system-wide PM transitions to sleep states.  It assumes that the analogous
  * pm_runtime_force_resume() will be used to resume the device.
+ *
+ * Do not use with DPM_FLAG_SMART_SUSPEND as this can lead to an inconsistent
+ * state where this function has called the ->runtime_suspend callback but the
+ * PM core marks the driver as runtime active.
  */
 int pm_runtime_force_suspend(struct device *dev)
 {