Add support for multi-field default sorts

The original implementation of `Meta.sort` only allowed a default sort to use one field. By always making `Meta.sort` iterable, the default sort can support multiple fields, just like when calling `sort()` on a `QuerySet`.

Setting `Meta.sort` to an iterable at creation time means that `Model.find()` doesn't need to perform the check itself each time.
Update Meta.sort documentation

With the changes made to `Meta.sort` in 40e4e04, the documentation needs this new information.