Clarify that DB and messaging system-specific conventions override common ones

[lmolkova]

Fixes #1235

Changes

• Cleans up confusing text left after Database and messaging: system-specific extensions should list all attributes they use #869 - the individual db/messaging system-specific conventions now list all applicable attributes, not additional ones
• Cleans up dynamodb doc - removes full list of db.systems.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.