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GH-34821: [DOC][ORC] Update documentation for ORC (#34822)

### Rationale for this change

The documentation of ORC is out of date. Now we have supported union and timestamp_instant types. It should be updated to reflect these changes.

### What changes are included in this PR?

Update the doc to add union and timestamp_instant to supported types.

### Are these changes tested?

Not neccessary.

### Are there any user-facing changes?

The doc itself is user-facing.
* Closes: #34821

Authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Will Jones <willjones127@gmail.com>