ticdc: add a note about schema

[hfxsd]

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What is changed, added or deleted? (Required)

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• v8.4 (TiDB 8.4 versions)
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• v8.2 (TiDB 8.2 versions)
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What is the related PR or file link(s)?

• This PR is translated from:ticdc: add a note about schema docs-cn#18545
• Other reference link(s):

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

[ti-chi-bot]

@3AceShowHand: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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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-8.1: #18818.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18819.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.3: #18820.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #18821.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #18822.