Doc: Hazelcast supports REPEATABLE_READ instead of READ_COMMITTED

[pveentjer]

The documentation should be updated. The documentation currently states that Hazelcast supports read committed isolation level. But when a map entry is read within a transaction, it automatically is locked, the actual isolation level is REPEATABLE_READ since no one can make a change to the data that has been read.

From a correctness point of view there is nothing wrong with over-delivering (we provide a higher isolation level than we promise). I think it is wise to explain what isolation level hazelcast is supporting.