docs: mention syncing as suggested by @rwgk

Author: henryiii

pybind11/setup_helpers.py

@@ -33,6 +33,12 @@
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 """
 
+# IMPORTANT: If you change this file in the pybind11 repo, also review
+# setup_helpers.pyi for matching changes.
+#
+# If you copy this file in, you don't
+# need the .pyi file; it's just an interface file for static type checkers.
+
 import contextlib
 import os
 import shutil

pybind11/setup_helpers.pyi

@@ -1,3 +1,6 @@
+# IMPORTANT: Should stay in sync with setup_helpers.py (mostly checked by CI /
+# pre-commit).
+
 from typing import Any, Iterator, Optional, Type, TypeVar, Union
 from types import TracebackType