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 <briannorris@chromium.org>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Link: https://patch.msgid.link/20250923150625.1.If11a14e33d578369db48d678395d0323bdb01915@changeid


[ rjw: Subject edits, changelog tweak ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>