Specify and document cache consistency guarantees

We can't simply use the consistency model defined by Java Specification and broaden it for whole cache (maybe the expression "can't" is too strong, but we definitely don't want to do that in some cases).

By consistency guarantees/model I mean mostly in which order are
writes allowed to be observed: and we can't boil it down to simply
causal, PRAM or any other consistency model as writes can be observed as non-atomic in Infinispan.

Infinispan documentation is quite scarce about that, the only trace I've
found is in Glossarry [2] "Infinispan has traditionally followed ACID
principles as far as possible, however an eventually consistent mode
embracing BASE is on the roadmap."