backport bug [ 1166582 ] IterableUserDict not in docs

Doc/lib/libuserdict.tex

@@ -27,12 +27,18 @@ contents are kept in a regular dictionary, which is accessible via the
 \member{data} attribute of \class{UserDict} instances.  If
 \var{initialdata} is provided, \member{data} is initialized with its
 contents; note that a reference to \var{initialdata} will not be kept, 
-allowing it be used for other purposes.
+allowing it be used for other purposes. \note{For backward compatibility,
+instances of \class{UserDict} are not iterable.}
+\end{classdesc}
+
+\begin{classdesc}{IterableUserDict}{\optional{initialdata}}
+Subclass of \class{UserDict} that supports direct iteration (e.g.
+\code{for key in myDict}).
 \end{classdesc}
 
 In addition to supporting the methods and operations of mappings (see
-section \ref{typesmapping}), \class{UserDict} instances provide the
-following attribute:
+section \ref{typesmapping}), \class{UserDict} and
+\class{IterableUserDict} instances provide the following attribute:
 
 \begin{memberdesc}{data}
 A real dictionary used to store the contents of the \class{UserDict}