document all input & output types

[sarahschwartz]

Currently the spec book defines 3 types of transaction inputs, however it seems there is a more granular level of input types that may be worth documenting for security researchers and chain analysts:

pub enum Input {
    CoinSigned(CoinSigned),
    CoinPredicate(CoinPredicate),
    Contract(Contract),
    MessageCoinSigned(MessageCoinSigned),
    MessageCoinPredicate(MessageCoinPredicate),
    MessageDataSigned(MessageDataSigned),
    MessageDataPredicate(MessageDataPredicate),
}

I'm not sure if there are also more granular levels of output types, but if there are we should document those as well.