docs: document TiFlash NullEQ pushdown

[windtalker]

What is changed, added, or deleted? (Required)

Update the TiFlash pushdown calculations documentation to cover the following support:

• TiFlash supports pushing down the NULL-safe equality operator (<=>) as an expression.
• TiFlash MPP HashJoin supports NULL-safe equi-join conditions that use <=>.

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

Tips for choosing the affected version(s):

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)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 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)

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

• Related code change PR links (if applicable): planner: support NullEQ pushdown to TiFlash tidb#71006
• This PR is translated from: docs: document TiFlash NullEQ pushdown docs#23926
• Other reference link(s):

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

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 46c3a8f and 30b21f4.

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• tiflash/tiflash-supported-pushdown-calculations.md

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📝 Walkthrough

Walkthrough

文档更新 TiFlash 的下推计算说明。HashJoin 支持列表新增空值安全型等值连接。数学函数运算符列表新增 <=>。

Changes

TiFlash 下推计算文档

Layer / File(s)	Summary
更新下推支持说明 tiflash/tiflash-supported-pushdown-calculations.md	HashJoin 支持列表新增使用 <=> 的空值安全型等值连接。数学函数运算符列表新增 <=>。

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~3 minutes

Change: Other

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	标题“docs: document TiFlash NullEQ pushdown”准确概括了文档变更，明确指出 TiFlash 的 NullEQ（<=>）下推支持。
Description check	✅ Passed	描述完整说明了变更内容、适用版本、相关代码 PR、翻译来源和 AI 参与情况。必填部分已填写，内容与变更一致。

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create a new PR

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