export_selected_objects example doesn't work when selecting all

[dmbst32@…]

In ​Actions that provide intermediate pages the export_selected_objects example is not correct. In a case where the "select all x objects" is marked, we expect the queryset to be all objects that match the user's filter. But request.POST.getlist(admin.ACTION_CHECKBOX_NAME) gives us only the objects checked off, not all objects.

[anonymous]

This is #15742.