Document optional 'task'/'asyncgen' fields in call_exception_handler (#21735)

Author: ShaneHarvey

Doc/library/asyncio-eventloop.rst

@@ -1184,10 +1184,13 @@ Allows customizing how exceptions are handled in the event loop.
    * 'message': Error message;
    * 'exception' (optional): Exception object;
    * 'future' (optional): :class:`asyncio.Future` instance;
+   * 'task' (optional): :class:`asyncio.Task` instance;
    * 'handle' (optional): :class:`asyncio.Handle` instance;
    * 'protocol' (optional): :ref:`Protocol <asyncio-protocol>` instance;
    * 'transport' (optional): :ref:`Transport <asyncio-transport>` instance;
-   * 'socket' (optional): :class:`socket.socket` instance.
+   * 'socket' (optional): :class:`socket.socket` instance;
+   * 'asyncgen' (optional): Asynchronous generator that caused
+                            the exception.
 
    .. note::