docs: fix illogical wording for HASH/KEY partitioned tables and DROP PARTITION

[yahonda]

What is changed, added or deleted? (Required)

best-practices/tidb-partitioned-tables-best-practices.md said:

Although global indexes can slow down `DROP PARTITION` operations, Hash and Key partitioned tables do not support `DROP PARTITION`. Therefore, global indexes are a practical solution because these partitions are rarely truncated.

This is self-contradictory: Hash and Key partitioned tables don't support `DROP PARTITION` at all (a hard constraint), so framing it as "these partitions are rarely truncated" misstates a total lack of support as a matter of low frequency. Reworded to state that the DROP-PARTITION performance trade-off simply doesn't apply to these tables, since the operation isn't available on them.

Found while reviewing the Japanese translation of this file (which faithfully mirrored this EN wording) — flagged by a CodeRabbit review on the JA PR.

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 wording-clarity 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.

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• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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• best-practices/tidb-partitioned-tables-best-practices.md

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