Explain why computed ES|QL chart values cannot filter or drill down

## Summary

ES|QL XY, partition, and heat map charts now explain when a value cannot be used for filtering or drilldowns because EVAL or STATS created the field at query time. XY and partition legends keep Filter for value and Filter out value visible but disabled with an explanation, while affected chart tooltips show the same limitation; renamed columns backed by index fields remain actionable.

**Why this needs docs:** Published ES|QL and drilldown pages say the action is unavailable, but the PR intentionally keeps some actions visible and disabled and adds explanatory feedback.

## Resources

- PR [#269722](https://github.com/elastic/kibana/pull/269722) — [ES|QL viz] Filters and drill downs on xy, heatmap and partition charts: provide feedback when cannot apply filter or drill downs when the value is computed
- Product issue: https://github.com/elastic/kibana/issues/260185

Screenshots from PR


## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.5.0 |
| **Serverless** | Jun 22–Jun 26 |
| **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

[Lens visualizations using ES|QL queries](https://www.elastic.co/docs/explore-analyze/visualize/esorql) > Add drilldowns to an ES|QL visualization
- **What the docs say:** The page says that values produced by EVAL or STATS are not backed by an index field, so the drilldown option is not available when users select those columns or series.
- **What to add:** Update the limitation to explain that XY and partition chart actions remain visible but disabled with a reason, affected tooltips also explain the limitation, and renamed columns backed by an index field remain actionable. Applies from 9.5.0 and in serverless.

[Add drilldowns](https://www.elastic.co/docs/explore-analyze/dashboards/drilldowns) > Requirements
- **What the docs say:** The requirements state that computed fields such as ES|QL EVAL or STATS results cannot trigger drilldowns and that the drilldown option is not available for them.
- **What to add:** Clarify that supported ES|QL charts provide feedback when a computed value cannot trigger a drilldown, including disabled legend actions on XY and partition charts and an explanatory tooltip message. Applies from 9.5.0 and in serverless.