[Telemetry SWX] add missing comment in mojom file

Add the missing comment in
chromeos/components/telemetry_extension_ui/mojom/diagnostics_service.mojom.

This is to make the RunBatteryHealthRoutine() method documentation
consistent with
chromeos/services/cros_healthd/public/mojom/cros_healthd.mojom.

Bug: b:171327161
Change-Id: I49ee24348138b7505ede83d900c9f48d1d238fc7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523191


Reviewed-by: Oleh Lamzin <lamzin@google.com>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Mahmoud Gawad <mgawad@google.com>
Cr-Commit-Position: refs/heads/master@{#824897}