Document PyFrozenSet_New() optimization

Author: vstinner

Doc/c-api/dict.rst

@@ -542,6 +542,10 @@ Frozen dictionary objects
 
    Create an empty dictionary if *iterable* is ``NULL``.
 
+   .. impl-detail::
+      If *iterable* is a :class:`frozendict`, but not a :class:`frozendict`
+      subclass, return the same object unmodified.
+
 
 Ordered dictionaries
 ^^^^^^^^^^^^^^^^^^^^