ticdc: update `log-file` explanation

[overvenus]

First-time contributors' checklist

• [x] I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

Update log-file explanation.

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)
• [x] v6.0 (TiDB 6.0 versions)
• [x] v5.4 (TiDB 5.4 versions)
• [x] v5.3 (TiDB 5.3 versions)
• [x] v5.2 (TiDB 5.2 versions)
• [x] v5.1 (TiDB 5.1 versions)
• [x] v5.0 (TiDB 5.0 versions)
• [ ] v4.0 (TiDB 4.0 versions)
• [ ] v3.1 (TiDB 3.1 versions)
• [ ] v3.0 (TiDB 3.0 versions)
• [ ] v2.1 (TiDB 2.1 versions)

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

• Other reference link(s): close https://github.com/pingcap/tiflow/issues/4880

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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[ti-chi-bot]

@overvenus: PR needs rebase.

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[ti-chi-bot[bot]]

@overvenus: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	19bcb4ad242141ed6f9bf82bb1ec2cf8a9dfd650	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.