ARROW-11165: [Rust][DataFusion] Document Postgres as standard SQL dialect
ARROW-11165: [Rust][DataFusion] Document Postgres as standard SQL dialect

PROPOSAL Document postgres as the target SQL / function dialect and rationale for this choice. I will also send an email to the dev mailing list soliciting feedback

There are several comments and more discussion on https://github.com/apache/arrow/pull/9108

Closes #9127 from alamb/alamb/ARROW-11165-dialect

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

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