Document wxJoystickEvent event changes

In particular, list the incompatible change in the corresponding section
and explain how to update the existing code.

docs/changes.txt

@@ -54,6 +54,12 @@ Changes in behaviour not resulting in compilation errors
 - wxEVT_AUINOTEBOOK_PAGE_CHANGED event is now sent after changing the page,
   as expected, and not before doing it.
 
+- wxJoystickEvent::GetButtonChange() now returns "1 << N" for the events
+  generated by the button number N under all platforms, whereas it used to
+  return just "N" under Linux and macOS. Use the new GetButtonOrdinal() to
+  update the existing code if necessary.
+
+
 Changes in behaviour which may result in build errors
 -----------------------------------------------------
 
@@ -131,6 +137,7 @@ All (GUI):
 - Add wxGrid::SetCornerLabelValue() (Pavel Kalugin).
 - Add strikethrough support for fonts defined in XRC.
 - Add wxDisplay::GetPPI().
+- Add wxJoystickEvent::GetButtonOrdinal() (Mick Phillips).
 
 wxGTK: