docs: clarify HYPO_INDEX scope in index advisor

[doc-claw-bot]

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)

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.

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

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[LGTM Timeline notifier]

Timeline:

• 2026-07-17 10:18:15.081539439 +0000 UTC m=+968081.117634485: ☑️ agreed by dveeden.