Transaction guide - Add not about setting cache level when using TX, and pitfalls when using XA

See PROD-864

The comment about why camel-jms is changed to not cache when transacted is enabled.

We should update the Transaction Guide, to indicate this.

If you are not using XA, then you can speedup performance by setting the cacheLevel to CONSUMER.
If you are using XA then keep it as is, otherwise you may end up with rollbacks that doesn't work properly.

I will update the Camel JMS wiki page
http://camel.apache.org/jms