releases: fix the invisible index testing steps in the v8.0.0 release notes

[yahonda]

What is changed, added or deleted? (Required)

In the v8.0.0 release notes, the tidb_opt_use_invisible_indexes feature description says:

With this feature, you can create a new index and test its performance by making the index visible first, and then modifying the system variable in the current session for testing without affecting other sessions.

"Making the index visible first" contradicts the feature. A visible index can be selected by the optimizer in every session, so the test would affect other sessions. tidb_opt_use_invisible_indexes exists so that an invisible index can be tested in the current session only:

• The system variables table in the same file says the variable "controls whether the optimizer can select invisible indexes for query optimization in the current session".
• The Chinese source (docs-cn, releases/release-8.0.0.md) says 「可以先将索引创建为不可见索引，然后通过修改该系统变量在当前会话中测试新索引的性能，而不影响其他会话」, that is, "first create the index as an invisible index, and then test the new index's performance in the current session by modifying the system variable, without affecting other sessions".

This PR changes the sentence to "you can create a new index as an invisible index first, and then test its performance by modifying the system variable in the current session without affecting other sessions."

This was found while reviewing the Japanese translation (#24152), which mirrors the EN text. This PR targets release-8.5 so that the weekly EN→JA sync carries the fix to the Japanese docs.

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)

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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
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• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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