[#75691732] Document vcloud-core public methods

> According to semver http://semver.org/ we should document the API
> (lower case should). We should document this by adding Yard
> documentation to public methods. See [#73962736] for an example.
I've actually said "look at the integration tests for this module" now, rather than duplicating the list of types that the function could return (which may change in future).