diff --git a/sphinx/builders/__init__.py b/sphinx/builders/__init__.py
index 252851ef4c3..61acef37e63 100644
--- a/sphinx/builders/__init__.py
+++ b/sphinx/builders/__init__.py
@@ -586,7 +586,9 @@ def write(
 
         if self.parallel_ok:
             # number of subprocesses is parallel-1 because the main process
-            # is busy loading doctrees and doing write_doc_serialized()
+            # is busy loading doctrees and doing write_doc_serialized();
+            # in case all extension support parallel_post_transform_safe
+            # the main process does nothing
             self._write_parallel(sorted(docnames),
                                  nproc=self.app.parallel - 1)
         else: