Add notes pointing out that these classes are kept for backward compatibility

and suggeest that new code that does not require compatibility with older
Python versions subclass dictionary, list, or str.

Doc/lib/libuserdict.tex

@@ -4,6 +4,11 @@
 \declaremodule{standard}{UserDict}
 \modulesynopsis{Class wrapper for dictionary objects.}
 
+\note{This module is available for backward compatibility only.  If
+you are writing code that does not need to work with versions of
+Python earlier than Python 2.2, please consider subclassing directly
+from the built-in \class{dictionary} type.}
+
 This module defines a class that acts as a wrapper around
 dictionary objects.  It is a useful base class for
 your own dictionary-like classes, which can inherit from
@@ -38,6 +43,11 @@ class.
 \modulesynopsis{Class wrapper for list objects.}
 
 
+\note{This module is available for backward compatibility only.  If
+you are writing code that does not need to work with versions of
+Python earlier than Python 2.2, please consider subclassing directly
+from the built-in \class{list} type.}
+
 This module defines a class that acts as a wrapper around
 list objects.  It is a useful base class for
 your own list-like classes, which can inherit from
@@ -92,6 +102,13 @@ methods which need to be provided in that case.
 \moduleauthor{Peter Funk}{pf@artcom-gmbh.de}
 \sectionauthor{Peter Funk}{pf@artcom-gmbh.de}
 
+\note{This \class{UserString} class from this module is available for
+backward compatibility only.  If you are writing code that does not
+need to work with versions of Python earlier than Python 2.2, please
+consider subclassing directly from the built-in \class{str} type
+instead of using \class{UserString} (there is no built-in equivalent
+to \class{MutableString}).}
+
 This module defines a class that acts as a wrapper around string
 objects.  It is a useful base class for your own string-like classes,
 which can inherit from them and override existing methods or add new