bpo-42230: Improve asyncio documentation regarding accepting sets vs iterables (GH-23073)

People call wait() and as_completed() with various non-set iterables,
a list should be the most common but there are others as well[1].

Considering typeshed also documents wait()[2] and as_completed()[3]
as accepting arbitrary iterables I think it's a good idea to document
the status quo better.

[1] https://github.com/aio-libs/aiokafka/pull/672
[2] https://github.com/python/typeshed/blob/620989bac572f30349b95590ebe81a73ce0fe862/stdlib/3/asyncio/tasks.pyi#L161
[3] https://github.com/python/typeshed/blob/620989bac572f30349b95590ebe81a73ce0fe862/stdlib/3/asyncio/tasks.pyi#L40