Clarify the distinction between the different algorithms ActiveMQ provides for assigning grouped messages to consumers

Issue

ActiveMQ provides a message grouping mechanism that attempts to assign messages in the same group to the same consumer. In this way messages can maintain their ordering within the group.

The allocation of groups to consumers is controlled by algorithms implemented in the selectable Java classes SimpleMessageGroupMap, MessageGroupHashBucket, and CachedMessageGroupMap. There is little documentation about the characteristics of these different algorithms, and it is unclear which to use in a particular situation.