Update documentation of Github.create_issue

copy and paste from Repository.create_issue.

The docstring of Github.create_issue and  Repository.create_issue still differs, but at least the important things should be fixed now.

Perhaps there is a way to keep them in sync without the need of copy and paste? For example only document params `owner` and `repository` and include a link to Repositroy.create_issue. As I'm very new to github3.py I did not realize that Github.create_issue is only a shorthand for Repository.create_issue.