ticdc : async exec ddl doc update (#14989)

[ti-chi-bot]

This is an automated cherry-pick of #14989

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• v7.4 (TiDB 7.4 versions)
• v7.3 (TiDB 7.3 versions)
• v7.2 (TiDB 7.2 versions)
• v7.1 (TiDB 7.1 versions)
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What is the related PR or file link(s)?

• This PR is translated from: ticdc : async exec ddl doc update docs-cn#14996
• Other reference link(s):

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

[qiancai]

/approve

[ti-chi-bot]

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