Resolve TODO comments for unexpected event warnings

Replace 5 'TODO: Should this be an error?' comments with descriptive
explanations noting these can happen during replay or duplicate event
delivery. The warning logs are already correctly implemented for all
event types (timerFired, taskCompleted, taskFailed,
subOrchestrationInstanceCompleted, subOrchestrationInstanceFailed).

Author: YunchuWang

durabletask/worker.py

@@ -1437,7 +1437,7 @@ def process_event(
                 timer_id = event.timerFired.timerId
                 timer_task = ctx._pending_tasks.pop(timer_id, None)
                 if not timer_task:
-                    # TODO: Should this be an error? When would it ever happen?
+                    # This can happen during replay or if a duplicate event is delivered
                     if not ctx._is_replaying:
                         self._logger.warning(
                             f"{ctx.instance_id}: Ignoring unexpected timerFired event with ID = {timer_id}."
@@ -1491,7 +1491,7 @@ def process_event(
                 task_id = event.taskCompleted.taskScheduledId
                 activity_task = ctx._pending_tasks.pop(task_id, None)
                 if not activity_task:
-                    # TODO: Should this be an error? When would it ever happen?
+                    # This can happen during replay or if a duplicate event is delivered
                     if not ctx.is_replaying:
                         self._logger.warning(
                             f"{ctx.instance_id}: Ignoring unexpected taskCompleted event with ID = {task_id}."
@@ -1506,7 +1506,7 @@ def process_event(
                 task_id = event.taskFailed.taskScheduledId
                 activity_task = ctx._pending_tasks.pop(task_id, None)
                 if not activity_task:
-                    # TODO: Should this be an error? When would it ever happen?
+                    # This can happen during replay or if a duplicate event is delivered
                     if not ctx.is_replaying:
                         self._logger.warning(
                             f"{ctx.instance_id}: Ignoring unexpected taskFailed event with ID = {task_id}."
@@ -1560,7 +1560,7 @@ def process_event(
                 task_id = event.subOrchestrationInstanceCompleted.taskScheduledId
                 sub_orch_task = ctx._pending_tasks.pop(task_id, None)
                 if not sub_orch_task:
-                    # TODO: Should this be an error? When would it ever happen?
+                    # This can happen during replay or if a duplicate event is delivered
                     if not ctx.is_replaying:
                         self._logger.warning(
                             f"{ctx.instance_id}: Ignoring unexpected subOrchestrationInstanceCompleted event with ID = {task_id}."
@@ -1578,7 +1578,7 @@ def process_event(
                 task_id = failedEvent.taskScheduledId
                 sub_orch_task = ctx._pending_tasks.pop(task_id, None)
                 if not sub_orch_task:
-                    # TODO: Should this be an error? When would it ever happen?
+                    # This can happen during replay or if a duplicate event is delivered
                     if not ctx.is_replaying:
                         self._logger.warning(
                             f"{ctx.instance_id}: Ignoring unexpected subOrchestrationInstanceFailed event with ID = {task_id}."