[Document] Why int32() offset type is used for DenseUnionArray?

[andishgar]

Describe the usage question you have. Please include as many useful details as possible.

According to the Apache Arrow specification, the dense union's offset type is int32. However, as noted, this limits arrays to 2 billion elements. Why not use int64, or is it possible to adopt a dual int32/int64 approach like dictionaries?

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