[FR] FR-018: add flow direction predict document

[zhangfane]

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• [X] I had searched in the issues and found no similar feature requirement.

Description

Detail the flow direction judgment and correction logic.

Use case

No response

Related issues

No response

Are you willing to submit a PR?

• [ ] Yes I am willing to submit a PR!

Code of Conduct

• [X] I agree to follow this project's Code of Conduct