update documentation for gpt-2
fixes a few incorrect details in the gpt-2 documentation.

one remaining thing, all of the models return an extra `presents` variable that I'm not quite sure what it is, so there's a ... in the doc. if you tell me what to put there I can put it there, or you can do it yourself.
Hi @joelgrus, you are right, the docstring were lagging a lot.
All the information is in the `README.py`, more specifically [these sections detailing the API of the GPT2 models](https://github.com/huggingface/pytorch-pretrained-BERT#14-gpt2model) but I forgot to update the docstrings. Do you want to have a look and tell me if it's detailed enough for the docstrings also?<|||||>yes, that's great, I made those changes. (I also feel kind of dumb for not looking at the docs docs.)

sorry about the trailing whitespace changes in the README.md, my editor removes those automatically.<|||||>Thanks Joel!