asyncio: SSL transports now clear their reference to the waiter

* Rephrase also the comment explaining why the waiter is not awaken immediatly.
* SSLProtocol.eof_received() doesn't instanciate ConnectionResetError exception
  directly, it will be done by Future.set_exception(). The exception is not
  used if the waiter was cancelled or if there is no waiter.
diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
index 97f9add..67973f1 100644
--- a/Lib/asyncio/unix_events.py
+++ b/Lib/asyncio/unix_events.py
@@ -301,7 +301,7 @@
         self._loop.add_reader(self._fileno, self._read_ready)
         self._loop.call_soon(self._protocol.connection_made, self)
         if waiter is not None:
-            # wait until protocol.connection_made() has been called
+            # only wake up the waiter when connection_made() has been called
             self._loop.call_soon(waiter._set_result_unless_cancelled, None)
 
     def __repr__(self):
@@ -409,7 +409,7 @@
 
         self._loop.call_soon(self._protocol.connection_made, self)
         if waiter is not None:
-            # wait until protocol.connection_made() has been called
+            # only wake up the waiter when connection_made() has been called
             self._loop.call_soon(waiter._set_result_unless_cancelled, None)
 
     def __repr__(self):