Update queue and rate_limit documentation

This pull request adds some notes about specifying a queue directly in a task, and some notes about how `rate_limit` works.  It also corrects an arithmetic error in the `rate_limit` example.  These were some gotchas that I ran into recently while working out an issue at work using a celery tasks on an API that strictly enforces a "one request every six seconds" rate limit.

The "worker instance" terminology was taken from [this celery-users discussion about `rate_limit`](https://groups.google.com/forum/#!msg/celery-users/NB0WPb6cpiU/JN07glB40pMJ).  I couldn't find a term for "concurrent thing running tasks", and "worker instance" was the term used in that discussion. If that's not the correct terminology, let me know, and I'll fix it.

Thanks!