Doesn't need a new interface, more doc is enough.

Closes (partially) libp2p/go-libp2p#999.

transport/transport.go

@@ -81,14 +81,9 @@ type Transport interface {
 	// See the Network interface for an explanation of how this is used.
 	// TODO: Make this a part of the go-multiaddr protocol instead?
 	Proxy() bool
-}
-
-// ClosableTransport is for transport that run a service beyond a listener
-// (overlay networks, ...).
-type ClosableTransport interface {
-	Transport
-
-	// Close is called when the libp2p node doesn't need the transport anymore.
+
+	// If your transport implements `io.Closer` Close will be called 
+	// when libp2p doesn't need it anymore.
 	// (libp2p closing, transport unhooked from a node, ...)
 	//
 	// Once a Close is called you can safely assume no more Dial or Listen are
@@ -98,7 +93,6 @@ type ClosableTransport interface {
 	// (ex: transport already closed or transport never started).
 	//
 	// No more than one Close will ever be issued on an instance.
-	Close() error
 }
 
 // Listener is an interface closely resembling the net.Listener interface. The