bpo-33649: First asyncio docs improvement pass (GH-9142)

Rewritten/updated sections:

* Event Loop APIs
* Transports & Protocols
* Streams
* Exceptions
* Policies
* Queues
* Subprocesses
* Platforms
diff --git a/Doc/library/asyncio.rst b/Doc/library/asyncio.rst
index b076b7d..0d58a94 100644
--- a/Doc/library/asyncio.rst
+++ b/Doc/library/asyncio.rst
@@ -50,7 +50,8 @@
    :maxdepth: 3
 
    asyncio-eventloop.rst
-   asyncio-eventloops.rst
+   asyncio-policy.rst
+   asyncio-platforms.rst
    asyncio-task.rst
    asyncio-protocol.rst
    asyncio-stream.rst
@@ -58,6 +59,7 @@
    asyncio-sync.rst
    asyncio-queue.rst
    asyncio-dev.rst
+   asyncio-exceptions.rst
 
 .. seealso::