docs: clarify HYPO_INDEX scope in index advisor (#23304)

[ti-chi-bot]

This is an automated cherry-pick of #23304

What is changed, added or deleted? (Required)

Users who see HYPO_INDEX written in the /*+ ... */ syntax might assume that a hypothetical index is a conventional optimizer hint. They might then look for HYPO_INDEX in the Optimizer Hints reference and be unsure why it is not listed there. The existing Index Advisor description does not clearly distinguish the hypothetical index being evaluated from the HYPO_INDEX hint used to define it.

This PR updates index-advisor.md for TiDB Cloud to:

• distinguish a hypothetical index from the special-purpose HYPO_INDEX hint that defines it
• clarify that HYPO_INDEX takes effect only in EXPLAIN statements and does not create a physical index
• introduce the example as an EXPLAIN use case, consistent with the SQL shown

These changes help users understand how HYPO_INDEX relates to optimizer hints and when to use it to evaluate an index design before creating a real index.

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

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

• This PR is translated from:
• Other reference link(s):
  • https://docs.pingcap.com/tidbcloud/index-advisor/#hypothetical-indexes
  • https://docs.pingcap.com/tidbcloud/optimizer-hints/
  • pingcap/tidb@ef0a3eb

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

Summary by CodeRabbit

• Documentation
  • Clarified how to use the /*+ HYPO_INDEX(...) */ syntax in EXPLAIN statements.
  • Updated explanations of hypothetical indexes, including their use in “What-If” analysis and experimentation.
  • Refined the introductory text for the hypothetical index example.

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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 c8064bb and b38dcd4.

📒 Files selected for processing (1)

• index-advisor.md

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📝 Walkthrough

Walkthrough

The hypothetical indexes documentation was revised to explain the /*+ HYPO_INDEX(...) */ syntax in EXPLAIN statements and update the accompanying example introduction.

Changes

Hypothetical index documentation

Layer / File(s)	Summary
EXPLAIN syntax documentation index-advisor.md	The “Hypothetical indexes” section now emphasizes using HYPO_INDEX within EXPLAIN and describes the example as an EXPLAIN statement using a hypothetical index.

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

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the doc clarification about HYPO_INDEX scope in index advisor.
Description check	✅ Passed	The description covers the change, affected version, related links, and context well enough for the template.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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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