#5199: make warning about vars() assignment more visible.

Doc/library/functions.rst

@@ -1352,8 +1352,12 @@ available.  They are listed here in alphabetical order.
    Without arguments, return a dictionary corresponding to the current local symbol
    table.  With a module, class or class instance object as argument (or anything
    else that has a :attr:`__dict__` attribute), returns a dictionary corresponding
-   to the object's symbol table.  The returned dictionary should not be modified:
-   the effects on the corresponding symbol table are undefined. [#]_
+   to the object's symbol table.
+
+   .. warning::
+
+      The returned dictionary should not be modified:
+      the effects on the corresponding symbol table are undefined. [#]_
 
 
 .. function:: xrange([start,] stop[, step])