Add a missing docstring

Lib/collections.py

+'''This module implements specialized container datatypes providing
+alternatives to Python’s general purpose built-in containers, dict,
+list, set, and tuple.
+
+* namedtuple   factory function for creating tuple subclasses with named fields
+* deque        list-like container with fast appends and pops on either end
+* Counter      dict subclass for counting hashable objects
+* OrderedDict  dict subclass that remembers the order entries were added
+* defaultdict  dict subclass that calls a factory function to supply missing values
+
+'''
+
 __all__ = ['Counter', 'deque', 'defaultdict', 'namedtuple', 'OrderedDict']
 # For bootstrapping reasons, the collection ABCs are defined in _abcoll.py.
 # They should however be considered an integral part of collections.py.