Documented feature doesn't exist: rlp.sedes.sedes_list

The documentation goes over the process of inferring which sedes class to use when serializing an object, [on this page](http://pyrlp.readthedocs.io/en/latest/tutorial.html#sedes-inference). It mentions that the function `rlp.infer_sedes` checks `rlp.sedes.sedes_list` to find a sedes object which can handle serializing whatever you are trying to encode. But the code for `infer_sedes` never checks any list like this, and the list `rlp.sedes.sedes_list` doesn't even exist! Not sure why this discrepancy between the code and the docs exist, thought I'd make an issue to point it out though.