Update relevant JMS docs to include the 'text' message class

[rbs4ba]

For the JMSSource and JMSSink operators, the 'text' message class was added a few years ago, after the initial implementation of the toolkit. With the addition of the 'text' message class, the documents were not updated to reflect that addition.

The documents should correctly list the 'text' message class as an available option.

[schubon]

Added some sentences in v5.3.9