ticdc: add a note about schema

[hfxsd]

First-time contributors' checklist

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

What is changed, added or deleted? (Required)

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

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For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
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• v7.1 (TiDB 7.1 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))

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

• This PR is translated from:
• Other reference link(s):https://github.com/pingcap/docs-cn/pull/15998/files https://github.com/pingcap/docs-cn/pull/15115/files

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

[hfxsd]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18550.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18551.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.3: #18552.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #18553.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #18554.