Improve documentation on segments

Hi,

Please improve the documentation on gmcast.segment. The current documentation ('It defines the segment to which the node belongs. ') is correct, but:

• Segments mean that galera protocol traffic is only redirected to 1 node in each segment, and then relayed to the other nodes in the same segment. This saves cross-datacenter network traffic at the expense of some extra latency.
• State transfers are also, preferably but not exclusively, taken from the same segment. If there are no nodes available in the same segment it will do SST from other node

Thanks!
Michaël