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] 620989bac5/stdlib/3/asyncio/tasks.pyiGH-L161
[3] 620989bac5/stdlib/3/asyncio/tasks.pyiGH-L40
(cherry picked from commit 3d86d090dc)

Co-authored-by: Jakub Stasiak <jakub@stasiak.at>

Doc/library/asyncio-task.rst

@@ -504,10 +504,10 @@ Waiting Primitives
                             return_when=ALL_COMPLETED)
 
    Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
-   set concurrently and block until the condition specified
+   iterable concurrently and block until the condition specified
    by *return_when*.
 
-   The *aws* set must not be empty.
+   The *aws* iterable must not be empty.
 
    Returns two sets of Tasks/Futures: ``(done, pending)``.
 
@@ -593,9 +593,9 @@ Waiting Primitives
 .. function:: as_completed(aws, \*, loop=None, timeout=None)
 
    Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
-   set concurrently.  Return an iterator of coroutines.
+   iterable concurrently.  Return an iterator of coroutines.
    Each coroutine returned can be awaited to get the earliest next
-   result from the set of the remaining awaitables.
+   result from the iterable of the remaining awaitables.
 
    Raises :exc:`asyncio.TimeoutError` if the timeout occurs before
    all Futures are done.