PEP 749: Add note on signature of __annotate__ (#3831)

peps/pep-0749.rst

@@ -540,6 +540,24 @@ an instance of a private stringizer class is used instead of ``ForwardRef``.
 After evaluation completes, the implementation of the FORWARDREF format
 converts these internal objects into ``ForwardRef`` objects.
 
+Signature of ``__annotate__`` functions
+---------------------------------------
+
+:pep:`649` specifies the signature of ``__annotate__`` functions as:
+
+  ``__annotate__(format: int) -> dict``
+
+However, using ``format`` as a parameter name could lead to collisions
+if an annotation uses a class named ``format``. The parameter should be
+positional-only and have a name that cannot be a legal identifier in order
+to avoid this problem.
+
+The current implementation uses the name ``.format`` with a leading
+dot, but the exact name should be considered an implementation detail
+and cannot be relied upon.
+
+The documentation may still use the name ``format`` for simplicity.
+
 Backwards Compatibility
 =======================