Add comments on cleanup/teardown phases.
Add comments on cleanup/teardown phases.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=229209449

GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker