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>

Documentation/ABI/testing/sysfs-devices-power

@@ -260,3 +260,12 @@ Description:
 
 		This attribute has no effect on system-wide suspend/resume and
 		hibernation.
+
+What:		/sys/devices/.../power/runtime_status
+Date:		April 2010
+Contact:	Rafael J. Wysocki <rjw@rjwysocki.net>
+Description:
+		The /sys/devices/.../power/runtime_status attribute contains
+		the current runtime PM status of the device, which may be
+		"suspended", "suspending", "resuming", "active", "error" (fatal
+		error), or "unsupported" (runtime PM is disabled).