The command line is just a wrapper for the JSON-RPC API, so you can consider this as the specification:
http://www.multichain.com/developers/json-rpc-api/
Also note that the first line output by multichain-cli on the command line shows you the exact JSON-RPC request sent.
One of the few tasks remaining before MultiChain 1.0 enters beta is to finalize all the error code, so please bear in mind that some of these may change. But not the API request parameters.