clarify documentation on replication.
* Fix [gh #95](https://github.com/m6w6/libmemcached/issues/95):
MEMCACHED_CALLBACK_GET_FAILURE and MEMCACHED_BEHAVIOR_BUFFER_REQUESTS
clarify documentation on replication.
* Fix [gh #95](https://github.com/m6w6/libmemcached/issues/95):
MEMCACHED_CALLBACK_GET_FAILURE and MEMCACHED_BEHAVIOR_BUFFER_REQUESTS