Include "instances of most classes" in a warning about mutable objects

as default values of function/method parameters.

Doc/tut/tut.tex

@@ -1388,8 +1388,9 @@ will print \code{5}.
 
 \strong{Important warning:}  The default value is evaluated only once.
 This makes a difference when the default is a mutable object such as a
-list or dictionary.  For example, the following function accumulates
-the arguments passed to it on subsequent calls:
+list, dictionary, or instances of most classes.  For example, the
+following function accumulates the arguments passed to it on
+subsequent calls:
 
 \begin{verbatim}
 def f(a, L=[]):