update ticdc failed changefeed gc mechanism description

[charleszheng44]

First-time contributors' checklist

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What is changed, added or deleted? (Required)

update the description of failed changefeed gc mechanism introduced by #8646

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

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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).

• master (the latest development version)
• v7.2 (TiDB 7.2 versions)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.6 (TiDB 6.6 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)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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Do your changes match any of the following descriptions?

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

[lilin90]

/lgtm

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-13 04:14:35.987082571 +0000 UTC m=+59072.401686635: ☑️ agreed by hfxsd.
• 2023-06-13 04:51:01.050637934 +0000 UTC m=+61257.465242010: ☑️ agreed by lilin90.

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[hfxsd]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hfxsd

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.0: #14209.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #14210.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #14211.