#6880: add reference to classes section in exceptions section, which comes earlier.

Doc/tutorial/errors.rst

@@ -245,9 +245,10 @@ re-raise the exception::
 User-defined Exceptions
 =======================
 
-Programs may name their own exceptions by creating a new exception class.
-Exceptions should typically be derived from the :exc:`Exception` class, either
-directly or indirectly.  For example::
+Programs may name their own exceptions by creating a new exception class (see
+:ref:`tut-classes` for more about Python classes).  Exceptions should typically
+be derived from the :exc:`Exception` class, either directly or indirectly.  For
+example::
 
    >>> class MyError(Exception):
    ...     def __init__(self, value):