Update the doc for read replica configuration

The doc for read replica is somewhat outdated： there is no ‘hbase.region.replica.replication.memstore.enabled’ in the code now， which is replaced by per-table property `REGION_MEMSTORE_REPLICATION`, and if we set `REGION_MEMSTORE_REPLICATION` to false,`hbase.region.replica.wait.for.primary.flush` will be ignored.