[cv] document Run.Submission field.
[cv] document Run.Submission field.

R=qyearsley, yiwzhang

Change-Id: Iaa772bf8e10b028d9c396c8987390dc33dcfb952
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2888968
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go