tidb-lightning: unify idle-worker wording with the Grafana panel table

[yahonda]

What is changed, added or deleted? (Required)

tidb-lightning/monitor-tidb-lightning.md describes the lightning_idle_workers gauge's name label values twice in the same document:

• Grafana panel table (Row 4: Quota, "Idle workers" panel): Number of unused \table-concurrency`, normally 0 until the end of process`
• Metrics reference (lightning_idle_workers labels): the remainder of \table-concurrency`, normally 0 until the end of the process`

Both describe the exact same value (the trailing clause is otherwise identical, word for word, for all 4 labels: table/index/region/io). Checked against the Chinese source (docs-cn/tidb-lightning/monitor-tidb-lightning.md), which uses the identical phrase 未使用的 X 的数量 ("number of unused X") in both places — so this is not an intentional distinction, just inconsistent EN wording introduced independently in two different PRs (#1455 and #1535) by the same author, 15 days apart, back in 2019.

This PR unifies the metrics-reference wording to match the already-established Grafana-table phrasing (Number of unused X), and drops the stray "the" in "until the end of the process" to exactly match the Grafana table's "until the end of process" for full consistency.

This will also let the Japanese translation express both sites the same way, instead of forcing two different Japanese renderings ("未使用の数" vs "の余り") for what is the same underlying value.

Which TiDB version(s) do your changes apply to? (Required)

• v8.5 (TiDB 8.5 versions)

What is the related PR or file link(s)?

• This PR is translated from: N/A (English-source wording fix, not a translation)
• Other reference link(s): tools/tidb-lightning: update docs for 3.0.2 and master #1455, tools/tidb-lightning: update TiDB Lightning FAQ and troubleshooting #1535 (the two commits that introduced each wording), docs-cn tidb-lightning/monitor-tidb-lightning.md (Chinese source showing both sites use the same wording)

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Clarified that the lightning_idle_workers metric reports unused workers for table, index, region, and I/O operations.

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📝 Walkthrough

Walkthrough

The lightning_idle_workers metric documentation now describes table, index, and region values as unused-worker counts. The io description retains its existing default and disk-speed interpretation.

Changes

Metric Documentation

Layer / File(s)	Summary
Update idle worker label descriptions tidb-lightning/monitor-tidb-lightning.md	The documentation changes label descriptions from concurrency remainders to unused-worker counts. Existing defaults and the io disk-speed interpretation remain unchanged.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~2 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to b209a

This documentation-only update does not affect runtime behavior and is ready to merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: it unifies the idle-worker wording in the TiDB Lightning documentation with the Grafana panel table.
Description check	✅ Passed	The description explains the documentation change, affected TiDB version, reference links, AI involvement, and scope checklist. It provides sufficient context for review.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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