docs: fix schema_unused_indexes source table and restart-scope wording

[yahonda]

What is changed, added or deleted? (Required)

best-practices/index-management-best-practices.md's "How schema_unused_indexes works" section said the view is derived from TIDB_INDEX_USAGE and filters out indexes with zero activity "since the last TiDB restart" (implying a single cluster-wide restart).

The actual view definition (see sys-schema/sys-schema-unused-indexes.md) is:

CREATE OR REPLACE VIEW sys.schema_unused_indexes AS
  SELECT
    table_schema as object_schema,
    table_name as object_name,
    index_name
  FROM information_schema.cluster_tidb_index_usage
  WHERE ...
  GROUP BY table_schema, table_name, index_name
  HAVING
    sum(last_access_time) is null;

This reads from CLUSTER_TIDB_INDEX_USAGE (which aggregates usage across all TiDB instances), not from the single-instance TIDB_INDEX_USAGE. It flags an index as unused only when none of the cluster's instances have recorded access since that instance's own last restart — not a single cluster-wide restart event.

Found while reviewing the Japanese translation of this file (which faithfully mirrored the inaccurate EN wording) — flagged by a CodeRabbit review on the JA PR, verified against the real view definition before filing this fix.

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

• 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)

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

• This PR is translated from: N/A (English factual-accuracy fix)
• Other reference link(s): i18n(ja): fix mistranslations and dropped particles across the best-practices/ directory #23888 (comment) (CodeRabbit finding on the JA translation PR)

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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Reviewing files that changed from the base of the PR and between 6acba5d and 3a21f13.

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• best-practices/index-management-best-practices.md

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