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>