docs: document TiFlash NullEQ pushdown

[windtalker]

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

Document that TiFlash supports pushing down the NULL-safe equality operator (<=>) as an expression and as an equi-join condition for MPP HashJoin.

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

• Related code change PR links (if applicable): planner: support NullEQ pushdown to TiFlash tidb#71006
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• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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Summary by CodeRabbit

• Documentation
  • Clarified the supported join types for pushdown calculations, including NULL-safe equi joins and specific non-equi joins.
  • Added the <=> operator to the documented list of supported numeric functions and operators.

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

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📝 Walkthrough

Walkthrough

The TiFlash pushdown documentation now lists NULL-safe equi joins using <=> and includes <=> in the supported numeric functions and operators.

Changes

TiFlash pushdown documentation

Layer / File(s)	Summary
Document join and operator support tiflash/tiflash-supported-pushdown-calculations.md	The HashJoin conditions now include NULL-safe Equi Join using <=>. The supported numeric functions and operators now include <=>.

Priority: ⬇️ Low

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

Change: Other

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description includes the required change summary, affected version, related code PR, AI involvement, and change-category checklist. It is complete and relevant.
Title check	✅ Passed	The title clearly and concisely identifies the documentation change for TiFlash NULL-safe equality pushdown.
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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