Since the Page Move dialog no longer allows page ordering, instructions should be provided

It was raised by Sarah during blitz testing that since page ordering can no longer be specified in the new Move Page dialog, we should provide instructions on how to order pages.

2) Given the above point, I think we should add some text to the UI explaining that you can only select the parent page here, not the order amongst pages at the same position in the hierarchy. We should also tell them where they can do that – i.e. if it's via the "Tree" view on the Browse Space "Pages" tab, tell them how to get there.