Document failover version

Author: dandavison

openapi/openapiv2.json

@@ -4570,7 +4570,7 @@
         "version": {
           "type": "string",
           "format": "int64",
-          "title": "TODO: What is this? Appears unused by SDKs"
+          "description": "Failover version. Defines alternate history branches that can arise in multi-cluster\nconfigurations (e.g. multi-region namespaces)."
         },
         "taskId": {
           "type": "string",

openapi/openapiv3.yaml

@@ -2965,7 +2965,9 @@ components:
           format: enum
         version:
           type: string
-          description: 'TODO: What is this? Appears unused by SDKs'
+          description: |-
+            Failover version. Defines alternate history branches that can arise in multi-cluster
+             configurations (e.g. multi-region namespaces).
         taskId:
           type: string
           description: 'TODO: What is this? Appears unused by SDKs'

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 can 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;