Fixed #18131 - Documented ContentTypeManager.get_for_id; thanks sir_sigurd for the report.

        probably won't ever need to call this method yourself; Django will call

        it automatically when it's needed.

    .. method:: get_for_id(id)

        Lookup a :class:`~django.contrib.contenttypes.models.ContentType` by ID.

        Since this method uses the same shared cache as

        :meth:`~django.contrib.contenttypes.models.ContentTypeManager.get_for_model`,

        it's preferred to use this method over the usual

        ``ContentType.objects.get(pk=id)``

    .. method:: get_for_model(model[, for_concrete_model=True])

        Takes either a model class or an instance of a model, and returns the