Currently, DeletePage() and RemovePage() functions don't send any events
when deleting the last page of the control and while this is
inconsistent with the behaviour when deleting the other pages, it seems
too dangerous to change this now, as the existing application code might
not expect getting page change events with the invalid page index.
So just document the current behaviour.
Ideal would be to add a unit test checking that this is really the case
under all platforms, for now it was just checked manually under MSW and
GTK.
Closes https://github.com/wxWidgets/wxWidgets/pull/1848
d83478e94d