Clarify that vars() doesn't place extra restrictions on __dict__.

Doc/library/functions.rst

@@ -1447,15 +1447,17 @@ available.  They are listed here in alphabetical order.
 
 .. function:: vars([object])
 
-   Without an argument, act like :func:`locals`.
+   Return the :attr:`__dict__` attribute for a module, class, instance,
+   or any other object with a :attr:`__dict__` attribute.
 
-   With a module, class or class instance object as argument (or anything else that
-   has a :attr:`__dict__` attribute), return that attribute.
+   Objects such as modules and instances have an updateable :attr:`__dict__`
+   attribute; however, other objects may have write restrictions on their
+   :attr:`__dict__` attributes (for example, new-style classes use a
+   dictproxy to prevent direct dictionary updates).
 
-   .. note::
-
-      The returned dictionary should not be modified:
-      the effects on the corresponding symbol table are undefined. [#]_
+   Without an argument, :func:`vars` acts like :func:`locals`.  Note, the
+   locals dictionary is only useful for reads since updates to the locals
+   dictionary are ignored.
 
 
 .. function:: xrange([start,] stop[, step])