Clarify `ReactiveTransactionManager` exception declarations

[jhoeller]

By design, ReactiveTransactionManager uses javadoc @throws clauses for indicating the exceptions that it propagates through the reactive pipeline returned from its methods. Unfortunately it also inherited Java method signature throws TransactionException declarations from PlatformTransactionManager which do not add anything to the signature here since TransactionException is a RuntimeException, and those declarations actually do not give a correct indication since the ReactiveTransactionManager methods are not expected to actually throw that exception type in a traditional sense. As a consequence, we'll drop those throws declarations from the method signature but keep the javadoc @throws indications around, in the ReactiveTransactionManager interface as well as the AbstractReactiveTransactionManager base class.

Also, along the lines of #30802, R2dbcTransactionManager actually propagates R2dbcException (a DataAccessException subclass) from commit/rollback which we should support in the ReactiveTransactionManager javadoc as well.