Add obsolescence note.

Doc/lib/libregsub.tex

@@ -7,6 +7,17 @@ regular expressions (see built-in module \code{regex}).
 
 Warning: these functions are not thread-safe.
 
+\strong{Obsolescence note:}
+This module is obsolete as of Python version 1.5; it is still being
+maintained because much existing code still uses it.  All new code in
+need of regular expressions should use the new \code{re} module, which
+supports the more powerful and regular Perl-style regular expressions.
+Existing code should be converted.  The standard library module
+\code{reconvert} helps in converting \code{regex} style regular
+expressions to \code{re} style regular expressions.  (For more
+conversion help, see the URL
+\file{http://starship.skyport.net/crew/amk/regex/regex-to-re.html}.)
+
 \renewcommand{\indexsubitem}{(in module regsub)}
 
 \begin{funcdesc}{sub}{pat\, repl\, str}

Doc/libregsub.tex

@@ -7,6 +7,17 @@ regular expressions (see built-in module \code{regex}).
 
 Warning: these functions are not thread-safe.
 
+\strong{Obsolescence note:}
+This module is obsolete as of Python version 1.5; it is still being
+maintained because much existing code still uses it.  All new code in
+need of regular expressions should use the new \code{re} module, which
+supports the more powerful and regular Perl-style regular expressions.
+Existing code should be converted.  The standard library module
+\code{reconvert} helps in converting \code{regex} style regular
+expressions to \code{re} style regular expressions.  (For more
+conversion help, see the URL
+\file{http://starship.skyport.net/crew/amk/regex/regex-to-re.html}.)
+
 \renewcommand{\indexsubitem}{(in module regsub)}
 
 \begin{funcdesc}{sub}{pat\, repl\, str}