Refine comment.

core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala

@@ -1190,6 +1190,9 @@ class DAGScheduler(
               // completed successfully from the perspective of the TaskSetManager, mark it as
               // no longer pending (the TaskSetManager may consider the task complete even
               // when the output needs to be ignored because the task's epoch is too small below).
+              // This can result in inconsistency between pending partitions and output locations
+              // of stage. When pending partitions is empty, the scheduler will check output
+              // locations, if there are missing, the stage will be resubmitted.
               shuffleStage.pendingPartitions -= task.partitionId
             }
             if (failedEpoch.contains(execId) && smt.epoch <= failedEpoch(execId)) {