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: NJohan Hovold <johan@kernel.org>
Reviewed-by: NUlf Hansson <ulf.hansson@linaro.org>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>