Add string concat item

Doc/whatsnew/whatsnew24.tex

@@ -815,6 +815,13 @@ the name \samp{None} is now a syntax error.
   the generated bytecode for list comprehensions and speeds them up
   by about a third.
 
+\item String concatenations in statements of the form \code{s = s +
+"abc"} and \code{s += "abc"} are now performed more efficiently in
+certain circumstances.  This optimization won't be present in other
+Python implementations such as Jython, so you shouldn't rely on it;
+using the \method{join()} method of strings is still recommended when
+you want to efficiently glue a large number of strings together.
+
 \end{itemize}
 
 The net result of the 2.4 optimizations is that Python 2.4 runs the