Mention that the MRO algorithm changes in 2.3

Doc/whatsnew/whatsnew22.tex

@@ -314,6 +314,9 @@ that state never getting saved.
 
 New-style classes follow a different algorithm that's a bit more
 complicated to explain, but does the right thing in this situation.
+(Note that Python 2.3 changes this algorithm to one that produces the
+same results in most cases, but produces more useful results for
+really complicated inheritance graphs.)
 
 \begin{enumerate}