docs: gthread is a sync worker

[jpmckinney]

https://github.com/benoitc/gunicorn/issues/1493#issuecomment-321461614

[benoitc]

gthread is not an asyncio worker but is an async worker. That should proabbly fiexed by adding it as a separate async worker. Though I am not sure why we document it. We should rather always use it using the threads setting. I am inclined to move that doc in the setting then.

[benoitc]

the gthreath work is not an asyncio worker but still an async worker though; i know it was a long time but maybe you can make the change to move that information to Async workers? I will keep it open .

[jpmckinney]

I've created a subsection of Async workers for Gthread workers.