Document the UserDict.popitem() override

Provide documentation for the ordering guarantees in UserDict.popitem()
in the method's docstring and in the Python docs.

Author: Andrew5057

Doc/library/collections.rst

@@ -1346,7 +1346,14 @@ attribute.
         A real dictionary used to store the contents of the :class:`UserDict`
         class.
 
+    :class:`!UserDict` instances also override the following method:
 
+    .. method:: popitem
+
+        Remove and return a ``(key, value)`` pair from the wrapped dictionary. Pairs are
+        returned in in the same order as :func:`.data.popitem`. (For the default
+        :meth:`dict.popitem`, this order is :abbr:`LIFO (last-in, first-out)`.) If the
+        dictionary is empty, raises a :exc:`KeyError`.
 
 :class:`UserList` objects
 -------------------------

Lib/collections/__init__.py

@@ -1257,6 +1257,13 @@ def copy(self):
     # This method has a default implementation in MutableMapping, but dict's
     # equivalent is last-in, first-out instead of first-in, first-out.
     def popitem(self):
+        """Remove and return a (key, value) pair as a 2-tuple.
+
+        Removes pairs in the same order as the wrapped mapping's popitem()
+        method. For dict objects (the default), that order is last-in,
+        first-out (LIFO).
+        Raises KeyError if the UserDict is empty.
+        """
         return self.data.popitem()