Update ValidationInterface() documentation to explicitly specify threading and memory model

src/validationinterface.h

@@ -53,6 +53,21 @@ void CallFunctionInValidationInterfaceQueue(std::function<void ()> func);
  */
 void SyncWithValidationInterfaceQueue();
 
+/**
+ * Implement this to subscribe to events generated in validation
+ *
+ * Each CValidationInterface() subscriber will receive event callbacks
+ * in the order in which the events were generated by validation.
+ * Furthermore, each ValidationInterface() subscriber may assume that
+ * callbacks effectively run in a single thread with single-threaded
+ * memory consistency. That is, for a given ValidationInterface()
+ * instantiation, each callback will complete before the next one is
+ * invoked. This means, for example when a block is connected that the
+ * UpdatedBlockTip() callback may depend on an operation performed in
+ * the BlockConnected() callback without worrying about explicit
+ * synchronization. No ordering should be assumed across
+ * ValidationInterface() subscribers.
+ */
 class CValidationInterface {
 protected:
     /**