PM: runtime: Documentation: ABI: Document time units for *_time

Many .../power/... time-related attributes have an "_ms" suffix and also
include language in their ABI description to make clear that their time
is measured in milliseconds. However, 'runtime_suspended_time' and
'runtime_active_time' have neither, and it takes me a nontrivial amount
of time to poke through the source to confirm that they are also
measured in milliseconds.

Update the doc with "millisecond" units.

Signed-off-by: Brian Norris <[e-mail]>
Reviewed-by: Dhruva Gole <[e-mail]>
Link: https://patch.msgid.link/20250923150625.1.If11a14e33d578369db48d678395d0323bdb01915@changeid
[ rjw: Subject edits, changelog tweak ]
Signed-off-by: Rafael J. Wysocki <[e-mail]>

Documentation/ABI/testing/sysfs-devices-power

···
 Date:		Jul 2010
 Contact:	Arjan van de Ven <[e-mail]>
 Description:
-		Reports the total time that the device has been active.
-		Used for runtime PM statistics.
+		Reports the total time that the device has been active, in
+		milliseconds. Used for runtime PM statistics.
 
 What:		/sys/devices/.../power/runtime_suspended_time
 Date:		Jul 2010
 Contact:	Arjan van de Ven <[e-mail]>
 Description:
-		Reports total time that the device has been suspended.
-		Used for runtime PM statistics.
+		Reports total time that the device has been suspended, in
+		milliseconds. Used for runtime PM statistics.
 
 What:		/sys/devices/.../power/runtime_usage
 Date:		Apr 2010