[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/+/2523191Reviewed-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}

chromeos/components/telemetry_extension_ui/mojom/diagnostics_service.mojom

@@ -55,6 +55,7 @@ interface DiagnosticsService {
   RunBatteryCapacityRoutine() => (RunRoutineResponse response);
 
   // Requests that the BatteryHealth routine is created and started on the
+  // platform. This routine checks the cycle count and percent wear of the
   // battery against inputs configured in cros_config. If no configuration data
   // is present in cros_config, the routine will fall back to fleet-wide default
   // values of 1000 for the maximum allowable cycle count and 50% for maximum