Is your feature request related to a problem or challenge? Please describe what you are trying to do.

arrow-row has comprehensive documentation on how types are encoded: https://github.com/apache/arrow-rs/blob/main/arrow-row/src/lib.rs

Describe the solution you'd like

With recent support for union encoding, we should update this documentation with union encoding format.

• Support Union data types for row format #8828

Describe alternatives you've considered

Additional context