Documentation about async callbacks

While facing with #1104, I realised that the callbacks in the websocket protocol may be coroutines. So, instead of

class Protocol(WebSocketServerProtocol):
    def onOpen(self):
        asyncio.ensure_future(coro(), loop=loop)

one could write

class Protocol(WebSocketServerProtocol):
    async def onOpen(self):
        await coro

The difference is there's no way to specify a loop, but the opportunity to use coroutines is very important yet undocumented.

Start by locating the documentation for WebSocketServerProtocol callbacks and the onOpen entry point, using issue #1104 for context. Document that callbacks may be async coroutines and show the coroutine form, including the limitation that it cannot specify an event loop; done means the supported behavior is clear in the relevant documentation.