[HUDI-4010] Improve docs on DynamoDB-based lock provider

[yihua]

Change Logs

This PR improves docs on DynamoDB-based lock provider to avoid confusion on setting the configs.

Impact

Risk level: none

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Contributor's checklist

• [ ] Read through contributor's guide
• [ ] Change Logs and Impact were stated clearly
• [ ] Adequate tests were added if applicable
• [ ] CI passed

[yihua]

cc @umehrot2 @rahil-c @CTTY