This CL updates the documentation for code coverage.

TBR=jbudorick@chromium.org

Bug: 974379
Change-Id: If9abde5ce40793a7a4d265703fb695126d8f0e56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663371
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671397}