Use IN and NOT IN subqueries in EVAL expressions

## Summary

The ES|QL editor now suggests and validates `IN` and `NOT IN` subqueries in EVAL, for example `EVAL col = keywordField IN (FROM other_index | KEEP keywordField)`. Published language docs still say IN subqueries work only in WHERE and are not supported in other commands. Confirm that Elasticsearch accepts IN in EVAL before rewriting that limitation.

**Why this needs docs:** Editor now treats EVAL as an IN subquery context, but published docs still limit IN subqueries to WHERE.

## Resources

- PR [#286735](https://github.com/elastic/kibana/pull/286735) — [ESQL] Enable in subquery in eval

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Aug 31–Sep 4 |
| **Feature flag** | None — active by default |

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*Created with [Docs Quest Scanner](https://github.com/florent-leborgne/docs-quest-scanner) by @florent-leborgne*

## Suggested edits

[Use ES|QL subqueries with IN and NOT IN](https://www.elastic.co/docs/reference/query-languages/esql/esql-in-subquery) > IN subqueries are only supported in the WHERE command
- **What the docs say:** An IN subquery can only appear in the WHERE command. It is not supported in other commands.
- **What to add:** If Elasticsearch accepts IN in EVAL, update this limitation and add an EVAL example such as `FROM index | EVAL col = field IN (FROM other | KEEP field)`. Also update the opening sentence that ties IN subqueries to WHERE only. Applies from 9.6.0 and in serverless.

[Nest ES|QL queries using subqueries](https://www.elastic.co/docs/reference/query-languages/esql/esql-subquery) > Learn more
- **What the docs say:** You can use subqueries in two places: in a FROM command, and in a WHERE command with IN or NOT IN.
- **What to add:** If IN in EVAL is supported, mention EVAL as a third place for IN and NOT IN subqueries. Applies from 9.6.0 and in serverless.