Recommend against Network's use in haddock summary

Per Tibbe's comment in Github issue #124:

    Aside: the whole Network module is unfortunately a bit ill-conceived
    (e.g. it assumes TCP only). We will keep it for backwards compatibility
    but new users should really use Network.Socket.

    (from https://github.com/haskell/network/issues/124#issuecomment-36513742)

I came here to suggest that Network.PortID's constructors not be exported, but it sounds like this might be a more general solution.