mention comparison operators changes

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42754 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -26,6 +26,12 @@ Changes in behaviour not resulting in compilation errors, please read this!
 Changes in behaviour which may result in compilation errors
 -----------------------------------------------------------
 
+- Some classes (notably wxBitmap and wxImage) don't define operator==() as it
+  used to work in somewhat unexpected way as it compared only internal pointers
+  and not the object data. The code using it will have to be updated:
+  * If the object was compared with wxNullXXX, use IsOk() method instead
+  * If valid object need to be compated, use IsRefTo() to reproduce the old
+    behaviour or change the code to avoid comparing bitmaps &c
 - wxFontData::GetColour() now returns a const colour.
 - wxDC objects can't be created directly now (this never worked, now it doesn't
   even compile).