Document that Code Quality needs a base report to compare to
From https://gitlab.com/gitlab-org/gitlab-ee/issues/2731#note_33453980
Spend some time as well before finding out "something to compare to" must exist for the widget to show up at all
@base_pipeline ||= project.pipelines.find_by(sha: merge_request_diff&.base_commit_sha)
We should document in https://docs.gitlab.com/ee/user/project/merge_requests/code_quality_diff.html.