[docs-agent] Note editor lacks autocomplete/validation for coordinator-mode lookup joins

[github-actions]

Summary

Adds one sentence to the "Cross-cluster and cross-project lookup joins" section of Use ES|QL in the Kibana UI noting that the ES|QL editor accepts the _coordinator: prefix for coordinator-mode lookup joins, but doesn't offer autocomplete suggestions or validation for coordinator-mode lookup indices. Without this note, the surrounding text ("suggests lookup mode indices and join condition fields") could mislead readers into thinking a coordinator-mode query with a nonexistent lookup index is valid or fully assisted by the editor.

The stack applies_to badge on this section was already ga 9.6 in the current source (updated in a same-day, unrelated commit), so no badge change was needed here.

Triage rationale

• Kibana PR elastic/kibana#280453 (merged) confirms the editor change: it strips/ignores the _coordinator: prefix for index-existence checks and validation, and explicitly skips lookup-index validation for coordinator targets (see validate.ts/sources.ts diffs and the added test skips lookup index existence validation for coordinator targets). No autocomplete entries are added for coordinator-mode targets.
• The underlying Elasticsearch feature (Coordinator lookup join for remote clusters elasticsearch#153804, "Coordinator lookup join for remote clusters") is merged and labeled v9.6.0, consistent with the issue's reported stack availability (v9.6.0) and the already-present stack: ga 9.6 badge on this page.
• The other suggested edit target, the LOOKUP JOIN reference page (esql-lookup-join.md), is sourced from the elastic/elasticsearch repository via the (redacted) cross-repo link, not docs-content`. Its "Planned" badge is out of scope for this repository and must be updated at the source.

Drafting sources

• Repository page: explore-analyze/query-filter/languages/esql-kibana.md
• Cumulative docs / applies_to guidance (contribute-docs/how-to/cumulative-docs)
• Published page for context: https://www.elastic.co/docs/explore-analyze/query-filter/languages/esql-kibana
• Implementation evidence: [ESQL] Support coordinator lookup join kibana#280453 (diff), [ES|QL] Support coordinator lookup joins in ES|QL editor kibana#279894, Coordinator lookup join for remote clusters elasticsearch#153804

Reader test

• Reader goal: a Kibana user writing ES|QL in the Discover/ES|QL editor understanding coordinator-mode LOOKUP JOIN support and why they might not see autocomplete or validation errors.
• 2 rounds. Round 1 flagged the wording as ambiguous about whether validation was "reduced" or "absent"; revised to explicitly state "won't see errors even if the index doesn't exist." Round 2 passed with no blocking comprehension gaps.

Closes #7626

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• expires on Aug 21, 2026, 3:55 PM UTC