query.ebnf: add coalesce rule, proofread and edit

Summary

• query.ebnf: add coalesce rule
• query.ebnf: proofread and edit

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• Docs follow-up:
  • docs#400 (closed) — add coalesce.svg
  • docs!646 (merged) — update expression.svg
• Changelog is updated
• Follow-up for !1425 (merged)