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GH-37720: [Format][Docs][FlightSQL] Document stateless prepared statements (#40243)

documents changes for stateless management of FlightSQL prepared
statement handles based on the design proposal described in
apache/arrow#37720

* GitHub Issue: #37720

PRs for language implementations:
* Rust: apache/arrow-rs#5433
* Go: apache/arrow#40311

Mailing list discussion:
https://lists.apache.org/thread/3kb82ypx99q96g84qv555l6x8r0bppyq

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Co-authored-by: David Li <li.davidm96@gmail.com>
Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>