Bug #1597576: mention that the new base64 api has been introduced in py2.4.

 (backport from rev. 52762)

Doc/lib/libbase64.tex

@@ -21,7 +21,7 @@ three alphabets.  The legacy interface provides for encoding and
 decoding to and from file-like objects as well as strings, but only
 using the Base64 standard alphabet.
 
-The modern interface provides:
+The modern interface, which was introduced in Python 2.4, provides:
 
 \begin{funcdesc}{b64encode}{s\optional{, altchars}}
 Encode a string use Base64.