Briefly describe the changes to private container classes

This is intentionally not very detailed, as we might find more container
classes to change in the future and listing all of the affected ones
here would require updating this list later.

docs/changes.txt

@@ -90,6 +90,14 @@ Changes in behaviour which may result in build errors
 - wxMotif and wxGTK1 ports have been removed, please use wxWidgets 3.2 if you
   still need them.
 
+- Several private container classes that never made part of wxWidgets public
+  API have been removed. If you used any of them (e.g. wxSimpleDataObjectList)
+  in your code, please switch to using std::vector<> or std::list<> instead.
+  Some other legacy "object array" classes (e.g. wxImageArray) still exist, but
+  are now similar to std::vector<> containing the objects -- they remain mostly
+  compatible with the previous wxWidgets versions, but now compare values, and
+  not pointers, in their Index() member function.
+
 - Generic wxSearchCtrl doesn't provide methods that make sense only for
   multiline text controls any longer, for consistency with the other ports.