cdc: update info message for ‘/api/v2/changefeeds/{changefeed_id}/synced’ (#10399)

[ti-chi-bot]

This is an automated cherry-pick of #10399

What problem does this PR solve?

Issue Number: ref #10289

What is changed and how it works?

update info message to make more clear

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

Please refer to [Release Notes Language Style Guide](https://pingcap.github.io/tidb-dev-guide/contribute-to-tidb/release-notes-style-guide.html) to write a quality release note.

If you don't think this PR needs a release note then fill it with `None`.

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
jenkins-ticdc/verify	6829928	link	true	/test verify
pull-cdc-integration-pulsar-test	6829928	link	true	/test cdc-integration-pulsar-test
pull-cdc-integration-mysql-test	6829928	link	true	/test cdc-integration-mysql-test
pull-cdc-integration-storage-test	6829928	link	true	/test cdc-integration-storage-test
pull-cdc-integration-kafka-test	6829928	link	true	/test cdc-integration-kafka-test

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[hongyunyan]

/close

[ti-chi-bot]

@hongyunyan: Closed this PR.

In response to this:

/close

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