Document ChainError Better

[TheBlueMatt]

From IRC:

<TamasBlummer> @BlueMatt: The SPV node I build in rust-bitcoin-spv would preferably store only a subset of the blockchain data, that concerns its own level1 or lightning wallet. How should it implement install_watch_tx (for the parameter txid) and get_chain_utxo? without full blockchain?
<BlueMatt> TamasBlummer: oops, should document that better, but you should return Err(ChainError::NotSupported)
<BlueMatt> https://docs.rs/lightning/0.0.6/lightning/chain/chaininterface/enum.ChainError.html
<BlueMatt> TamasBlummer: you can freely not support that function at all, or support it only partially, as you see fit

This should obviously be better documented so as to not result in questions.