Update documentation (#713)

## What do these changes do?

This PR updates the documentation to reflect the latest changes:
recommends to use the new `aclose()` method (ad2b1bd) and clarifies that
the slowdown "can be", but not necessarily "will be" (4a57895). In fact,
in no-wait tests Janus can now be comparable to standard queues, but in
worst cases where notification methods are often called, it can still
show significant performance slowdown, so "only when used as intended"
remains. In particular, in Culsans benchmarks Janus is still slower than
the naive solution.

## Are there changes in behavior for the user?

There are no behavior changes for users.

README.rst

@@ -69,8 +69,7 @@ Example
         fut = loop.run_in_executor(None, threaded, queue.sync_q)
         await async_coro(queue.async_q)
         await fut
-        queue.close()
-        await queue.wait_closed()
+        await queue.aclose()
 
 
     asyncio.run(main())
@@ -83,16 +82,16 @@ This library is built using a classic thread-safe design. The design is
 time-tested, but has some limitations.
 
 * Once you are done working with a queue, you must properly close it using
-  ``close()`` and ``wait_closed()``. This is because this library creates new
-  tasks to notify other threads. If you do not properly close the queue,
+  ``aclose()``. This is because this library creates new tasks to notify other
+  threads. If you do not properly close the queue,
   `asyncio may generate error messages
   <https://github.com/aio-libs/janus/issues/574>`_.
 * The library has acceptable performance only when used as intended, that is,
   for communication between synchronous code and asynchronous one.
   For sync-only and async-only cases, use queues from
   `queue <https://docs.python.org/3/library/queue.html>`_ and
   `asyncio <https://docs.python.org/3/library/asyncio-queue.html>`_ modules,
-  otherwise `the slowdown will be significant
+  otherwise `the slowdown can be significant
   <https://github.com/aio-libs/janus/issues/419>`_.
 * You cannot use queues for communicating between two different event loops
   because, like all asyncio primitives, they bind to the current one.