Clarify where the artifacts metadata .gz is generated

What does this MR do?

• Clarify where the artifacts metadata .gz is generated

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

When we updated the gitlab/spec/fixtures/ci_build_artifacts.zip, it took me a while to figure out how the metadata file was generated, gitlab/spec/fixtures/ci_build_artifacts_metadata.gz in order to update it. I eventually found this is done in GitLab workhorse, https://gitlab.com/gitlab-org/gitlab-workhorse/blob/dab2f932042719b80c148d62ff6c26394714e020/internal/artifacts/artifacts_upload.go#L26

I still don't know how to manually update it but at least I know how it happens now. Here is my quick attempt at a script but it doesn't work, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14399#note_42203431

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?