releases: add release notes for v8.4.0

[Oreoxmt]

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What is changed, added or deleted? (Required)

releases: add release notes for v8.4.0

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

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By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions))

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

• This PR is translated from: releases: add release notes for v8.4.0 docs#18829
• Other reference link(s):

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

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[MyonKeminta]

Suggested change

	TiDB 在向 PD 请求 TSO 时，会将一段时间内的请求汇总起来并以同步的方式进行批处理，以减少 RPC (Remote Procedure Call) 请求数量从而降低 PD 负载。对于延迟敏感的场景，这种同步模式的性能并不理想。
	在 v8.4.0 中，TiDB 新增 TSO 请求的异步批处理模式，并提供不同的并发能力。异步模式可以降低获取 TSO 的延迟，但可能会增加 PD 的负载。你可以通过 [`tidb_tso_client_rpc_mode`](/system-variables.md#tidb_tso_client_rpc_mode-从-v840-版本开始引入) 变量设定获取 TSO 的 RPC 模式。
	TiDB 在向 PD 请求 TSO 时，会将一段时间内的请求汇总起来并以串行的方式进行批处理，以减少 RPC (Remote Procedure Call) 请求数量从而降低 PD 负载。对于延迟敏感的场景，这种串行模式的性能并不理想。
	在 v8.4.0 中，TiDB 新增 TSO 请求的并行批处理模式，并提供不同的并发能力。并行模式可以降低获取 TSO 的延迟，但可能会增加 PD 的负载。你可以通过 [`tidb_tso_client_rpc_mode`](/system-variables.md#tidb_tso_client_rpc_mode-从-v840-版本开始引入) 变量设定获取 TSO 的 RPC 模式。

[ti-chi-bot]

@Oreoxmt: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	d69802d	link	true	/test pull-verify

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