Document new TSDB behaviour: accept older timestamps within the eligible write window.

### Overview

PR [#152716](https://github.com/elastic/elasticsearch/pull/152716) (targeting `9.5.0`) adds opt-in on-demand creation of past TSDS backing indices during writes. Several docs-content assertions remain correct by default but are incomplete or become misleading when the feature is enabled; the largest gap is that the "eligible write window" concept is not documented at all.

### TODOs

- [ ] id: fix-time-bound-tsds
content: "Revise time-bound-tsds.md: conditional rejection rule, eligible write window, backfill subsection (applies_to stack 9.5+)"
status: pending
- [ ] id: cross-link-setup-security
content: Update set-up-tsds.md accepted-time-range note and secure-tsds privileges for backfill
status: pending
- [ ] id: optional-tsds-overview
content: Add brief backfill mention to time-series-data-stream-tsds.md and advanced-topics nav
status: pending
- [x] id: coordinate-es-reference
content: Document cluster setting API key in elasticsearch PR before linking from docs-content
status: done (part of [#152716](https://github.com/elastic/elasticsearch/pull/152716))
- [x] id: optional-release-notes
content: Add 9.5/serverless release note snippet when feature merges
status: done (part of [#152716](https://github.com/elastic/elasticsearch/pull/152716))
- [ ] id: loading-historical
content: Add guide on how to load longer periods of data
status: pending
- [ ] id:dlm-downsampling-floodgate
content: Update the downsampling documentation in DLM to mention that there is a configurable "floodgate"
status: pending

Relates to: https://github.com/elastic/elasticsearch/pull/152716

TriageBot — Complete

### 2026-07-08

**Type:** documentation

**Section check:**
- ✅ Problem statement: New TSDB feature (eligible write window) is not documented; specific docs needing updates are identified.
- ✅ Proposed outcome / definition of done: TODO list with specific files and tasks (three pending, two done).

**Cross-references:**
- `elastic/elasticsearch#152716` — PR state could not be verified from this environment; referenced as the upstream implementation.

**Next step:** none — issue is complete.

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