Document failover version

Author: dandavison

temporal/api/history/v1/message.proto

@@ -767,7 +767,8 @@ message HistoryEvent {
     int64 event_id = 1;
     google.protobuf.Timestamp event_time = 2;
     temporal.api.enums.v1.EventType event_type = 3;
-    // TODO: What is this? Appears unused by SDKs
+    // Failover version. Defines alternate history branches that arise in multi-cluster configurations (e.g.
+    // multi-region namespaces).
     int64 version = 4;
     // TODO: What is this? Appears unused by SDKs
     int64 task_id = 5;