Add a comment about 3k migration

Doc/whatsnew/whatsnew26.tex

@@ -29,6 +29,9 @@ rationale, refer to the PEP for a particular new feature.
 
 % Large, PEP-level features and changes should be described here.
 
+% Should there be a new section here for 3k migration?
+% Or perhaps a more general section describing module changes/deprecation?
+% sets module deprecated
 
 %======================================================================
 \section{Other Language Changes}