Documentation: document AbstractServer, Server.sockets is specific to asyncio event loops

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I just fixed the issue #180 which documents the Server.sockets attribute.

AbstractServer has no sockets attribute nor constructor. Should we add a 
sockets attribute to AbstractServer? Here is a patch:
http://codereview.appspot.com/113040043

Or maybe we should drop this abstract server class? I like the idea of an 
interface for event loop. I'm not sure that an interface for server is useful. 
The class is exported in the asyncio namespace, but it is not documented.

By the way, I replaced AbstractServer with Server in the documentation. For 
event loops, BaseEventLoop is documented, not AbstractEventLoop.

Original issue reported on code.google.com by victor.s...@gmail.com on 11 Jul 2014 at 10:05