Database: clarify list of SQL `db.system`s

[lmolkova]

Changes

Following up on the WG discussion inspired by https://github.com/open-telemetry/semantic-conventions/issues/965

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] Change log entry added, according to the guidelines in When to add a changelog entry.
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• ~~schema-next.yaml updated with changes to existing conventions.~~