Document WS_EX_COMPOSITED impact on wxClientDC

This is a big and incompatible change but, arguably, is still worth
making because it makes wxMSW behaviour similar to that of wxOSX and
wxGTK when using Wayland.

Do document it prominently and also document MSWDisableComposited() as
it will probably end up being used in quite a lot of existing code.

docs/changes.txt

@@ -11,6 +11,14 @@ INCOMPATIBLE CHANGES SINCE 3.2.x:
 Changes in behaviour not resulting in compilation errors
 --------------------------------------------------------
 
+- wxMSW now uses double buffering by default, meaning that updating the
+  windows using wxClientDC doesn't work any longer, which is consistent with
+  the behaviour of wxGTK with Wayland backend and of wxOSX, but not with the
+  traditional historic behaviour of wxMSW (or wxGTK/X11). You may call
+  MSWDisableComposited() to restore the previous behaviour, however it is
+  strongly recommended to change your redrawing logic to avoid using wxClientDC
+  instead, as the code using it still won't work with wxGTK/wxOSX.
+
 - As first mentioned in 3.0 release notes, the value of wxTHREAD_WAIT_DEFAULT,
   used by wxThread::Delete() and Wait() by default, has changed from
   wxTHREAD_WAIT_YIELD to wxTHREAD_WAIT_BLOCK for safety and consistency.