ticdc: add description for pulsar topic config

[lidezhu]

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[LGTM Timeline notifier]

Timeline:

• 2024-10-11 09:43:48.127765253 +0000 UTC m=+5025.276675077: ☑️ agreed by hfxsd.
• 2024-10-11 09:43:49.768440915 +0000 UTC m=+5026.917350740: ☑️ agreed by hfxsd.

[hfxsd]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #18795.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18796.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18797.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.3: #18798.