docs: Refine description for field task_execution

PiperOrigin-RevId: 633294486

google/cloud/batch/v1alpha/task.proto

@@ -107,8 +107,18 @@ message StatusEvent {
 // This Task Execution field includes detail information for
 // task execution procedures, based on StatusEvent types.
 message TaskExecution {
-  // When task is completed as the status of FAILED or SUCCEEDED,
-  // exit code is for one task execution result, default is 0 as success.
+  // The exit code of a finished task.
+  //
+  // If the task succeeded, the exit code will be 0. If the task failed but not
+  // due to the following reasons, the exit code will be 50000.
+  //
+  // Otherwise, it can be from different sources:
+  // - Batch known failures as
+  // https://cloud.google.com/batch/docs/troubleshooting#reserved-exit-codes.
+  // - Batch runnable execution failures: You can rely on Batch logs for further
+  // diagnose: https://cloud.google.com/batch/docs/analyze-job-using-logs.
+  // If there are multiple runnables failures, Batch only exposes the first
+  // error caught for now.
   int32 exit_code = 1;
 
   // Optional. The tail end of any content written to standard error by the task