Add comment to KeyValueCacheRequest.
Add comment to KeyValueCacheRequest.

Sorry I forgot to add this in the previous CL.

Bug: catapult:#4461
Change-Id: I7b4dbf26b2fe27d086e3ea62c0e9f2368c229ef1
Reviewed-on: https://chromium-review.googlesource.com/c/1278833
Commit-Queue: Ben Hayden <d3ffe0e065c2328e497bb32a07f4dcb29138ef5b@chromium.org>
Reviewed-by: Sean McCullough <7caa448b00260c603c5227306703b63f5f75ff01@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult