Restore compatibility with 3.0-like DrawCaptionSelectionRect()

This function was not called any longer after 35a8d0f908 (Fix building
wxPropertyGrid with v3.0 compatibility enabled, 2023-03-22), which
silently broke any applications resulting on this to happen, so call it
again, even if it's rather painful and ugly to do.

Also add wxDEPRECATED_BUT_USED_INTERNALLY_MSG() and use it instead of the
plain wxDEPRECATED_MSG() for the deprecated overload of this function to
avoid warnings when calling it from wxWidgets itself.

Finally, document the change to DrawCaptionSelectionRect() as this
wasn't done back in c63b1604b3 (Use native renderer to draw rectangle
indicating that wxPG category property is selected., 2015-08-30) when
its signature was changed.

docs/changes.txt

@@ -73,6 +73,10 @@ Changes in behaviour not resulting in compilation errors
   available. If any locale using the given language is acceptable, the region
   must be left empty, e.g. just "fr" would use any available "fr_XX".
 
+- Deprecated wxPGCellRenderer::DrawCaptionSelectionRect() overload is not
+  called any longer by default, you need to explicitly enable 3.0 compatibility
+  or change your code to override the newer overload, taking a wxWindow pointer.
+
 
 Changes in behaviour which may result in build errors
 -----------------------------------------------------