Union typeIds width

[Paul Taylor]

I noticed the the DenseUnion docs[1] says the typeIds buffer is 8-bit 
signed integers, but in the flatbuffer schema[2] it's typed as int (and 
flatc generates a function that returns an Int32Array).

How are the other implementations treating this buffer, and should we 
update the docs or the flatbuffers schema?

Thanks,

Paul

1. https://arrow.apache.org/docs/format/Layout.html#dense-union-type

2. 
https://github.com/apache/arrow/blob/50bc9f49671afb56594910f49b9bf34e080a70e7/format/Schema.fbs#L92