Document TSDB backfill behaviour

[lcawl]

Summary

Addresses #7252

Core documentation

• time-bound-tsds.md — Conditional rejection when past-index creation is enabled; expanded rejection tip; new Eligible write window and Backfill past timestamps sections (9.5+); cluster settings, lifecycle origination date, security, and link to the loading guide.
• load-historical-tsds.md (new) — Step-by-step guide for greenfield bootstrap, in-window backfill, beyond-window historical stream pattern (stepper), cluster protection, and limitations.

Cross-links and nav

• set-up-tsds.md — Softer accepted-time-range note; auto_configure security requirement; next-steps link.
• time-series-data-stream-tsds.md — Backfill mentions in “When to use” and backing indices.
• advanced-topics-tsds.md + toc.yml — New page wired into navigation.

DLM floodgate

• run-downsampling.md — data_streams.lifecycle.downsampling.max_indices_in_progress (default 10).
• data-stream.md — Brief mention in lifecycle flow step 4.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Composer 2.5

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🔍 Preview links for changed docs

• manage-data/data-store/data-streams/advanced-topics-tsds.md
• manage-data/data-store/data-streams/failure-store.md
• manage-data/data-store/data-streams/load-historical-tsds.md
• manage-data/data-store/data-streams/reindex-tsds.md
• manage-data/data-store/data-streams/run-downsampling.md
• manage-data/data-store/data-streams/set-up-tsds.md
• manage-data/data-store/data-streams/time-bound-tsds.md
• manage-data/data-store/data-streams/time-series-data-stream-tsds.md
• manage-data/lifecycle/data-stream.md

[github-actions]

Elastic Docs Style Checker (Vale)

Summary: 9 suggestions found

💡 Suggestions (9): Optional style improvements. Apply when helpful.

File	Line	Rule	Message
manage-data/data-store/data-streams/load-historical-tsds.md	20	Elastic.WordChoice	Consider using 'deactivated, deselected, hidden, turned off, unavailable' instead of 'disabled', unless the term is in the UI.
manage-data/data-store/data-streams/time-bound-tsds.md	51	Elastic.WordChoice	Consider using 'deactivated, deselected, hidden, turned off, unavailable' instead of 'disabled', unless the term is in the UI.
manage-data/data-store/data-streams/time-bound-tsds.md	77	Elastic.Semicolons	Use semicolons judiciously.
manage-data/data-store/data-streams/time-bound-tsds.md	88	Elastic.Wordiness	Consider using 'all' instead of 'all of '.
manage-data/data-store/data-streams/time-bound-tsds.md	99	Elastic.WordChoice	Consider using 'deactivated, deselected, hidden, turned off, unavailable' instead of 'disabled', unless the term is in the UI.
manage-data/data-store/data-streams/time-bound-tsds.md	114	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.
manage-data/data-store/data-streams/time-bound-tsds.md	126	Elastic.WordChoice	Consider using 'deactivate, deselect, hide, turn off' instead of 'disable', unless the term is in the UI.
manage-data/data-store/data-streams/time-bound-tsds.md	126	Elastic.WordChoice	Consider using 'deactivated, deselected, hidden, turned off, unavailable' instead of 'disabled', unless the term is in the UI.
manage-data/lifecycle/data-stream.md	43	Elastic.WordChoice	Consider using 'run, start' instead of 'execute', unless the term is in the UI.

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