swarming: add comment for mail inbound_service
swarming: add comment for mail inbound_service

I wondered why we need this and found
crrev.com/42b735086e357696d34f695d9759afc8a7e61c96

Change-Id: I8498d6f7dd06472d205953a82b6746e9fc4e9c93
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2196004
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Reviewed-by: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py