Add Union encoding documentation

[EduardAkhmetshin]

Which issue does this PR close?

• Closes Update arrow-row documentation with Union encoding #9084.

What changes are included in this PR?

Documentation on union types encoding in https://arrow.apache.org/rust/arrow_row/struct.RowConverter.html.

Are these changes tested?

Yes.

Are there any user-facing changes?

Yes. https://arrow.apache.org/rust/arrow_row/struct.RowConverter.html will get updated.

[EduardAkhmetshin]

I tested union encoding myself, and the behaviour that I observed differed from the original proposal in #8828. I based my examples on the actual behaviour to the best of my knowledge. Please let me know if something is wrong.

[alamb]

@friendlymatthew -- can you help review this?

[alamb]

Thanks -- I went over this carefully and I think it looks correct and really nicely explained. Thank you @EduardAkhmetshin

cc @Jefffrey

[Jefffrey]

Would be good to mention reversing via negating the type_id here:

arrow-rs/arrow-row/src/lib.rs

Lines 1742 to 1747 in 2507946

	let type_id_byte = if opts.descending {
	!(type_id as u8)
	} else {
	type_id as u8
	};
	data[*offset] = type_id_byte;

[alamb]

Since I am trying to prepare the 57.2.0 I took the liberty of making this change directly to this PR in 08efd42. If I got it wrong or you would like other changes, I will be happy to make a follow on PR

[alamb]

Thanks again @EduardAkhmetshin @Jefffrey and @friendlymatthew