Specify that it's namespace failover version

Co-authored-by: Roey Berman <[email protected]>

Author: dandavison

temporal/api/history/v1/message.proto

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