Clarify meaning of Git ref in CI artifacts context

What does this MR do?

While discussing several CI artifacts related tickets recently, Support noted that our docs might not make it obvious that each tag counts as a ref for which an artifact copy is kept when the "keep latest" setting is active.

@nelbacha: Could you please review this as well? Did I capture the core of the observation with the suggested change?

Related issues

Example ticket and another (internal)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
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  • Request a review from the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• [-] Update the permissions table.
• [-] Link docs to and from the higher-level index page, plus other related docs where helpful.
• [-] Add the product tier badge accordingly.
• [-] Add GitLab's version history note(s).
• [-] Add/update the feature flag section.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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For more information about labels, see Technical Writing workflows - Labels.

For suggestions that you are confident don't need to be reviewed, change them locally and push a commit directly to save others from unneeded reviews. For example:

• Clear typos, like this is a typpo.
• Minor issues, like single quotes instead of double quotes, Oxford commas, and periods.

For more information, see our documentation on Merging a merge request.

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