tikv config: document new resource-control configs for v8.5.6

[mittalrishabh]

Add 11 new configuration parameters to the resource-control section introduced by TiKV PRs cherry-picked into the v8.5.6 release:

• bg-cpu-throttle-threshold / fg-cpu-throttle-threshold (CPU throttling)
• bg-compaction-pressure-threshold / bg-write-io-ceiling / bg-write-io-floor (I/O throttling)
• enable-fair-scheduling / enable-read-admission-control / enable-write-admission-control
• historical-usage-window-mins / baseline-burst-pct / admission-max-delayed-count

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[gemini-code-assist]

Code Review

This pull request adds documentation for several new TiKV configuration items introduced in v8.5.6 and v9.0.0, including CPU throttling thresholds, write I/O limits, fair scheduling, and admission control settings. The review feedback suggests style improvements across all these additions to adhere to the repository style guide, specifically by replacing passive voice with active voice, using the second person ("you") when addressing users, and standardizing formatting.

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[gemini-code-assist]

To adhere to the repository style guide, we should avoid passive voice and wordy phrasing. Let's rewrite this description to use active voice and more precise terminology.

Suggested change

+ Specifies the CPU utilization percentage threshold at which background task throttling begins. When CPU utilization reaches this value, TiKV starts to reduce the resource budget allocated to background tasks (such as compaction and garbage collection). The background task budget scales linearly from full down to zero as CPU utilization increases from this threshold toward [`fg-cpu-throttle-threshold`](#fg-cpu-throttle-threshold).

+ Specifies the CPU utilization percentage threshold at which background task throttling begins. When CPU utilization reaches this value, TiKV starts reducing the resource budget for background tasks (such as compaction and garbage collection). The background task budget scales linearly from 100% to 0% as CPU utilization increases from this threshold toward [`fg-cpu-throttle-threshold`](#fg-cpu-throttle-threshold).

References

1. Avoid unnecessary words and passive voice. ^(link)

[gemini-code-assist]

To adhere to the repository style guide, we should avoid passive voice. Let's rewrite this description to use active voice.

Suggested change

	+ Specifies the CPU utilization percentage threshold above which foreground traffic protection is fully activated. When CPU utilization reaches this value, background tasks are completely throttled to their minimum floor and the background utilization budget is capped at this value. This threshold must be greater than [`bg-cpu-throttle-threshold`](#bg-cpu-throttle-threshold).
	+ Specifies the CPU utilization percentage threshold above which TiKV fully activates foreground traffic protection. When CPU utilization reaches this value, TiKV completely throttles background tasks to their minimum floor and caps the background utilization budget at this value. This threshold must be greater than [`bg-cpu-throttle-threshold`](#bg-cpu-throttle-threshold).

References

1. Avoid passive voice overuse. ^(link)

[gemini-code-assist]

To adhere to the repository style guide, we should avoid passive voice and informal phrasing like 'ramps up'. Let's rewrite this description to use active voice and more formal terminology.

Suggested change

	+ Specifies the compaction pending bytes ratio threshold at which background write I/O throttling begins. Below this threshold, background write I/O ramps up toward [`bg-write-io-ceiling`](#bg-write-io-ceiling). At or above this threshold, background write I/O scales linearly down toward [`bg-write-io-floor`](#bg-write-io-floor) as compaction pressure approaches 100%.
	+ Specifies the compaction pending bytes ratio threshold at which background write I/O throttling begins. Below this threshold, background write I/O increases toward [`bg-write-io-ceiling`](#bg-write-io-ceiling). At or above this threshold, TiKV scales background write I/O linearly down toward [`bg-write-io-floor`](#bg-write-io-floor) as compaction pressure approaches 100%.

References

1. Avoid passive voice and use consistent, clear terminology. ^(link)

[gemini-code-assist]

To adhere to the repository style guide, we should avoid passive voice. Let's rewrite this description to use active voice.

Suggested change

	+ Specifies the maximum write I/O rate allowed for background tasks when compaction pressure is below [`bg-compaction-pressure-threshold`](#bg-compaction-pressure-threshold).
	+ Specifies the maximum write I/O rate for background tasks when compaction pressure is below [`bg-compaction-pressure-threshold`](#bg-compaction-pressure-threshold).

References

1. Avoid passive voice overuse. ^(link)

[gemini-code-assist]

To adhere to the repository style guide, we should avoid passive voice. Let's rewrite this description to use active voice.

Suggested change

	+ Specifies the minimum write I/O rate guaranteed to background tasks even at maximum compaction pressure. This floor prevents background tasks from being starved completely.
	+ Specifies the minimum write I/O rate that TiKV guarantees for background tasks even at maximum compaction pressure. This floor prevents background task starvation.

References

1. Avoid passive voice overuse. ^(link)

[gemini-code-assist]

To adhere to the repository style guide, we should avoid passive voice and write in the second person ('you') when addressing users. Let's rewrite this description to use active voice and second person.

Suggested change

	+ Controls whether to enable admission control for read requests. When enabled, read requests from over-baseline resource groups are delayed or rejected with `SchedTooBusy` when CPU utilization exceeds [`fg-cpu-throttle-threshold`](#fg-cpu-throttle-threshold). This configuration takes effect only when [`enable-fair-scheduling`](#enable-fair-scheduling) is also enabled.
	+ Controls whether to enable admission control for read requests. When you enable this feature, TiKV delays or rejects read requests from over-baseline resource groups with `SchedTooBusy` when CPU utilization exceeds [`fg-cpu-throttle-threshold`](#fg-cpu-throttle-threshold). This configuration takes effect only when you also enable [`enable-fair-scheduling`](#enable-fair-scheduling).

References

1. Avoid passive voice and write in second person ('you') when addressing users. ^(link)

[gemini-code-assist]

To adhere to the repository style guide, we should avoid passive voice and write in the second person ('you') when addressing users. Let's rewrite this description to use active voice and second person.

Suggested change

	+ Controls whether to enable admission control for write requests. When enabled, write requests from over-baseline resource groups are delayed or rejected with `SchedTooBusy` when CPU utilization exceeds [`fg-cpu-throttle-threshold`](#fg-cpu-throttle-threshold). This configuration takes effect only when [`enable-fair-scheduling`](#enable-fair-scheduling) is also enabled.
	+ Controls whether to enable admission control for write requests. When you enable this feature, TiKV delays or rejects write requests from over-baseline resource groups with `SchedTooBusy` when CPU utilization exceeds [`fg-cpu-throttle-threshold`](#fg-cpu-throttle-threshold). This configuration takes effect only when you also enable [`enable-fair-scheduling`](#enable-fair-scheduling).

References

1. Avoid passive voice and write in second person ('you') when addressing users. ^(link)