PM: runtime: Documentation: add runtime_status ABI document

/sys/devices/.../power/runtime_status is introduced by commit c92445fa
("PM / Runtime: Add sysfs debug files").  Then commit 0fcb4eef ("PM /
Runtime: Make runtime_status attribute not debug-only (v. 2)") made the
runtime_status attribute available without CONFIG_PM_ADVANCED_DEBUG.

This adds missing runtime_status ABI document.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>