Update migration-api-overview.md by adding notice on file-level auditing events

[zuojihong]

Add file-level auditing statement

Category

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[learn-build-service-prod]

Learn Build status updates of commit 9532d01:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/apis/migration-api-overview.md	✅Succeeded	View

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[andrewconnell]

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