[Coverge] Update code coverage doc to explain the workflow

This CL updates the code coverage doc to explain the workflow of
generating code coverage reports.

Change-Id: Ie79386eccccdf01be568ff52481d43ed95443b5c
Reviewed-on: https://chromium-review.googlesource.com/999817
Commit-Queue: Yuke Liao <[email protected]>
Reviewed-by: Max Moroz <[email protected]>
Reviewed-by: Martin Barbella <[email protected]>
Cr-Commit-Position: refs/heads/master@{#548883}
4 files changed