DistributedFlowRuleStore: synchronized -> Reader/Writer lock

fix for ONOS-195

Change-Id: I3e15104225878d1616fa790095695400bcc43697
diff --git a/core/api/src/main/java/org/onlab/onos/net/flow/FlowRuleStore.java b/core/api/src/main/java/org/onlab/onos/net/flow/FlowRuleStore.java
index 958dc91..bba5a3c 100644
--- a/core/api/src/main/java/org/onlab/onos/net/flow/FlowRuleStore.java
+++ b/core/api/src/main/java/org/onlab/onos/net/flow/FlowRuleStore.java
@@ -60,6 +60,7 @@
      * Stores a batch of flow rules.
      *
      * @param batchOperation batch of flow rules.
+     *           A batch can contain flow rules for a single device only.
      * @return Future response indicating success/failure of the batch operation
      * all the way down to the device.
      */