Add some syncdb deprecation stuff into the deprecation doc

* ``allow_syncdb`` on database routers will no longer automatically become

  ``allow_migrate``.

* The ``DatabaseCreation`` class on each database backend will be removed,

  and all table/schema editing will be moved to be via ``SchemaEditor`` instead.

* The legacy method of syncing apps without migrations will be removed,

  and migrations will become compulsory for all apps.

* All models will need to be defined inside an installed application or

  declare an explicit :attr:`~django.db.models.Options.app_label`.

  Furthermore, it won't be possible to import them before their application