Add warning in ctypes documentation for #16575 and #16576
diff --git a/Doc/library/ctypes.rst b/Doc/library/ctypes.rst
index 486c963..6f093fe 100644
--- a/Doc/library/ctypes.rst
+++ b/Doc/library/ctypes.rst
@@ -603,6 +603,13 @@
.. _ctypes-structureunion-alignment-byte-order:
+.. warning::
+
+ :mod:`ctypes` does not support passing unions or structures with bit-fields
+ to functions by value. While this may work on 32-bit x86, it's not
+ guaranteed by the library to work in the general case. Unions and
+ structures with bit-fields should always be passed to functions by pointer.
+
Structure/union alignment and byte order
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^