Documentation for the new clone method.

Even though I am aware that the comment lines in pygit2/__init__.py are longer than 79 characters, there's a reason for that.
If I break them, they'll show poorly in the documentation, and in my opinion better docs trump the 80 char requirement. If you think it's still better to have less than 80 characters in the comments, I'll gladly resubmit the changes at the expense of the documentation.