Add clarification in WFT started_event_id docstring (#330)

Author: Sushisource

temporal/api/workflowservice/v1/request_response.proto

@@ -257,7 +257,9 @@ message PollWorkflowTaskQueueResponse {
     // Will be zero if no task has ever started.
     int64 previous_started_event_id = 4;
     // The id of the most recent workflow task started event, which will have been generated as a
-    // result of this poll request being served.
+    // result of this poll request being served. Will be zero if the task
+    // does not contain any events which would advance history (no new WFT started).
+    // Currently this can happen for queries.
     int64 started_event_id = 5;
     // Starting at 1, the number of attempts to complete this task by any worker.
     int32 attempt = 6;