Simplify the doc; omit subclass special case

Author: vstinner

Doc/c-api/dict.rst

@@ -543,8 +543,8 @@ 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.
+      If *iterable* is a :class:`frozendict`, return the same object
+      unmodified.
 
 
 Ordered dictionaries