Add examples to Rust Docs for arrow::array::UnionArray

[matthewmturner]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I would like to see examples of how to use UnionArrays in the Rust docs (https://docs.rs/arrow/5.3.0/arrow/array/struct.UnionArray.html). The explanations are good, but examples would improve my understanding.

Describe the solution you'd like
Add examples for new and try_new`

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
I thought of this while I was looking into #654

[HaoYang670]

I'd like to try if no one has been doing it.