pg_upgrade: clarify C comment about Windows thread struct pointers

Backpatch to 9.3 to keep source trees consistent.

contrib/pg_upgrade/parallel.c

@@ -130,7 +130,7 @@ parallel_exec_prog(const char *log_file, const char *opt_log_file,
 			/* fork failed */
 			pg_log(PG_FATAL, "could not create worker process: %s\n", strerror(errno));
 #else
-		/* use first empty array element */
+		/* empty array element are always at the end */
 		new_arg = exec_thread_args[parallel_jobs - 1];
 
 		/* Can only pass one pointer into the function, so use a struct */
@@ -244,7 +244,7 @@ parallel_transfer_all_new_dbs(DbInfoArr *old_db_arr, DbInfoArr *new_db_arr,
 			/* fork failed */
 			pg_log(PG_FATAL, "could not create worker process: %s\n", strerror(errno));
 #else
-		/* use first empty array element */
+		/* empty array element are always at the end */
 		new_arg = transfer_thread_args[parallel_jobs - 1];
 
 		/* Can only pass one pointer into the function, so use a struct */
@@ -339,10 +339,10 @@ reap_child(bool wait_for_child)
 		thread_handles[thread_num] = thread_handles[parallel_jobs - 1];
 
 		/*
-		 * We must swap the arg struct pointers because the thread we just
-		 * moved is active, and we must make sure it is not reused by the next
-		 * created thread.	Instead, the new thread will use the arg struct of
-		 * the thread that just died.
+		 * Move last active thead arg struct into the now-dead slot,
+		 * and the now-dead slot to the end for reuse by the next thread.
+		 * Though the thread struct is in use by another thread, we can
+		 * safely swap the struct pointers within the array.
 		 */
 		tmp_args = cur_thread_args[thread_num];
 		cur_thread_args[thread_num] = cur_thread_args[parallel_jobs - 1];