ARROW-12075: [Rust][DataFusion] Add CTE + UNION ALL to supported list of SQL features
ARROW-12075: [Rust][DataFusion] Add CTE + UNION ALL to supported list of SQL features

@Dandandan  added the feature in https://github.com/apache/arrow/pull/9776, updating docs to keep pace

Closes #9795 from alamb/alamb/CTE

Authored-by: Andrew Lamb <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@nerdnetworks.org>
Signed-off-by: Andrew Lamb <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@nerdnetworks.org>

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