WaitIterator constructor typing is not match with documentation

[schaumb]

tornado.gen.WaitIterator constructor parameters are can be Awaitable-s based on documentation, but the constructor typing says only Future https://github.com/tornadoweb/tornado/blob/master/tornado/gen.py#L367

The Waititerator works as expected on tornado.locks.Event().wait(), but the mypy fails on it.

[bdarnell]

The typing is accurate; the docs are wrong. WaitIterator does not currently work with all awaitables, only Futures. (Event.wait returns a Future even though its typing says Awaitable)

However, I think it's better to make the code behave as documented than to change the docs to match the code.