As a follow-up of gh-22915, the purpose of this commit is to improve
Coroutines programmatic transaction API to make it more consistent with
the Java one and more idiomatic.
For suspending functions, this commit changes the
TransactionalOperator.transactional extension with a suspending lambda
parameter to a TransactionalOperator.executeAndAwait one which is
conceptually closer to TransactionalOperator.execute Java API so more
consistent.
For Flow, the TransactionalOperator.transactional extension is correct
but would be more idiomatic as a Flow extension.
This commit also adds code samples to the reference documentation.
Closes gh-23627