charset: update deal with invalid utf8 characters

[Oreoxmt]

What is changed, added or deleted? (Required)

https://github.com/pingcap/tidb/pull/61972 changed the behaviour of deal with invalid utf8 characters, we should update related 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 (in Chinese).

• [x] master (the latest development version)
• [ ] v9.0 (TiDB 9.0 versions)
• [ ] v8.5 (TiDB 8.5 versions)
• [ ] v8.4 (TiDB 8.4 versions)
• [ ] v8.3 (TiDB 8.3 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)

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

• This PR is translated from: https://github.com/pingcap/docs-cn/pull/20555
• Other reference link(s): https://github.com/pingcap/tidb/pull/61972

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

[ti-chi-bot[bot]]

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[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-07-18 08:22:36.762396483 +0000 UTC m=+2852009.485575465: :ballot_box_with_check: agreed by qiancai.

[Oreoxmt]

/cc @Defined2014