ConnectionInterface should extend DuplexStreamInterface + documentation

[clue]

Make sure the ConnectionInterface actually extends the DuplexStreamInterface introduced with react/stream v0.4.2. Thus, the main difference here is this:

-interface ConnectionInterface extends ReadableStreamInterface, WritableStreamInterface
+interface ConnectionInterface extends DuplexStreamInterface

Note that this is not a BC break because the DuplexStreamInterface actually extends both ReadableStreamInterface and WritableStreamInterface.

Also update the documentation to emphasize this is the main interface that is to be used to represent incoming connections.