PM / runtime: Document new pm_runtime_set_suspended() constraint

Since commit a8636c89 (PM / Runtime: Don't allow to suspend a
device with an active child) it is no longer permitted to set
RPM_SUSPENDED state for a device with active children unless
power.ignore_children is set.

Update the documentation to reflect this.
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/power/runtime_pm.txt

@@ -435,7 +435,8 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h:
       PM status to 'suspended' and update its parent's counter of 'active'
       children as appropriate (it is only valid to use this function if
       'power.runtime_error' is set or 'power.disable_depth' is greater than
-      zero)
+      zero); it will fail and return an error code if the device has a child
+      which is active and the 'power.ignore_children' flag is unset
 
   bool pm_runtime_active(struct device *dev);
     - return true if the device's runtime PM status is 'active' or its