bpo-41621: Document defaultdict's default_factory parameter (GH-21945)

It defaults to None and is positional only.

Doc/library/collections.rst

@@ -707,9 +707,9 @@ stack manipulations such as ``dup``, ``drop``, ``swap``, ``over``, ``pick``,
 :class:`defaultdict` objects
 ----------------------------
 
-.. class:: defaultdict([default_factory[, ...]])
+.. class:: defaultdict(default_factory=None, /, [...])
 
-    Returns a new dictionary-like object.  :class:`defaultdict` is a subclass of the
+    Return a new dictionary-like object.  :class:`defaultdict` is a subclass of the
     built-in :class:`dict` class.  It overrides one method and adds one writable
     instance variable.  The remaining functionality is the same as for the
     :class:`dict` class and is not documented here.

Misc/NEWS.d/next/Documentation/2020-08-24-13-35-04.bpo-41621.nqaw9G.rst

@@ -0,0 +1 @@
+Document that :class:`collections.defaultdict` parameter ``default_factory`` defaults to None and is positional-only.

Modules/_collectionsmodule.c

@@ -2241,7 +2241,7 @@ defdict_init(PyObject *self, PyObject *args, PyObject *kwds)
 }
 
 PyDoc_STRVAR(defdict_doc,
-"defaultdict(default_factory[, ...]) --> dict with default factory\n\
+"defaultdict(default_factory=None, /, [...]) --> dict with default factory\n\
 \n\
 The default factory is called without arguments to produce\n\
 a new value when a key is not present, in __getitem__ only.\n\