improve batching documentation

Email from galder
Aside from these questions, I think http://community.jboss.org/docs/DOC-14901 could do with having more information. Brian's http://community.jboss.org/wiki/BatchModeTransactionManager contains some info on what's the rationale behind batching and I suspect suspect some of that is useful in Infinispan as well.

On top of that, I think http://community.jboss.org/docs/DOC-14901 should answer:

• Are locks maintained in a batch?
• What happens at commit time in a local cache?
• What happens at commit time in a clustered cache? 1PC or 2PC
• What happens if a cache loader is involved? Are updates atomic on the cacheloader?
• When/Why would someone use batching instead of JTA transactions? And viceversa?
  ...etc