Describe the Jakarta Transactions 2.1 read-only feature

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**
https://github.com/jakartaee/platform/blob/main/specification/src/main/asciidoc/platform/TransactionManagement.adoc does not yet describe the [Transactions 2.1 read-only feature](https://github.com/jakartaee/transactions/issues/220).

**Describe the solution you'd like**
https://jakarta.ee/specifications/platform/11/jakarta-platform-spec-12.0#transaction-requirements should describe the Transactions 2.1 read-only feature as described in https://github.com/jakartaee/transactions/commit/a60daaa4e2231eb6f57ddf679e46455be5a18a15#diff-a7f1a8f7aa4fe67e98dd606d2752e6380bbc1d952dbd3e46907126923ba6125e and the final Jakarta Transactions 2.1 specification.

We need to explain how read-only transactions are expected to work in Jakarta EE 12+ applications so that users know how to use this new feature correctly.

**Describe alternatives you've considered**
none.

**Additional context**

https://www.eclipse.org/lists/messaging-dev/threads.html#00135 contains a Jakarta Messaging mailing list discussion about the Transactions read-only feature.

Start in specification/src/main/asciidoc/platform/TransactionManagement.adoc and locate the transaction-requirements section. Compare it with the Jakarta Transactions 2.1 specification, commit a60daaa4e2231eb6f57ddf679e46455be5a18a15, and the linked messaging-dev discussion. Done means the platform specification explains expected read-only transaction behavior for Jakarta EE 12+ applications.

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