Remove mentions of queue as valid gather arguments

In #2671 the option to use queues and iterators as arguments to certain distributed Client methods was removed. However, some documentation still mentions iterators and queues as valid arguments. For example:

https://docs.dask.org/en/latest/futures.html#distributed.Client.gather

I'm not sure if the mentioned PR removes support for iterat-**able** objects like lists or only generator/iterator objects. So I'm not sure exactly how the docstring needs to be updated. I also am not sure I have the time to make a PR this week. Maybe a good hacktoberfest issue? @mrocklin ?