Is there documentation related to the built-in multisig wallet contract in the Mist Wallet? I see there are many additional functions in this contract like changing / adding / removing owners etc. However, it's not clear how to use these other features though.
Smart contracts expose multiple functions as a public API. In general, to interact with any contract, its ABI (different from API) must be used to properly encode/decode messages to/from a contract. This is true regardless of function or event.
The Mist multisig wallet is based on the code at https://raw.githubusercontent.com/ethereum/dapp-bin/master/wallet/wallet.sol. However, this makes use of many Solidity language features, so understanding it is not as straightforward as reading a single contract's code.
Mist exposes most of the contract functionality via an HTML interface embedded in the application. If you want to arbitrarily interact with functions not provided in the default interface, you can use web3.js or some other high-level tool.