Review and improve documentation of arguments

[Stuart Mumford]

Currently there is two main parts of the API, the constructor to Download and the enqueue_file method, both of these accept many more kwargs that are in the function signature or docstring due to them getting passed down the tree into the partial functions that get executed in the loop etc. We should make sure that we are using clear keyword argument names and that it's possible to easily see all the possible arguments in the docs (and even better in function introspection).