explain-joins: reword "create ... on disk" as "spill ... to disk" (#23346)

[ti-chi-bot]

This is an automated cherry-pick of #23346

What is changed, added or deleted? (Required)

In the Hash Join section of explain-joins.md, one sentence describes the on-disk behavior as:

TiDB will attempt to use temporary storage, and might create the Build operator (used as part of the hash join) on disk.

"create the Build operator on disk" is confusing — the operator is not created on disk; its data is spilled to temporary storage. Later on the same page, the system-variables section already describes the same behavior with clearer wording:

TiDB will attempt to spill the Build operator of a hash join to disk to save memory.

This PR aligns the earlier sentence with that clearer, consistent wording ("spill ... to disk").

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

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)

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

N/A — documentation wording clarification.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Clarified the hash join runtime statistics documentation to explain that data from the Build side might be spilled to disk when the query memory limit is exceeded.

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• explain-joins.md

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📝 Walkthrough

Walkthrough

The hash join documentation now describes spilling data from the Build side to disk when tidb_mem_quota_query is exceeded.

Changes

Hash Join Documentation

Layer / File(s)	Summary
Runtime statistics wording explain-joins.md	The documentation replaces the description of creating the Build operator on disk with a description of spilling Build-side data to disk.

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

Merge Risk: ⚪ Minimal · up to 66787

This documentation-only update clarifies hash join disk spilling behavior without changing runtime behavior or configuration. It is ready to merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the documentation wording change from creating the Build operator on disk to spilling data to disk.
Description check	✅ Passed	The description explains the change and reason, identifies the affected TiDB version, and provides related-change information. Some optional template checkboxes are omitted, but the required content i…
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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