[#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.
Refer to where details of entity types documented

@annashipman suggested listing the types available, but I'd rather not
do that when there are already examples in the integration spec for
these methods.
Document boolean returns for vCloud operations

Technically these methods always return `true` if they return at all,
as that's the return value from `process_task` which is the last call
used by these methods.

If `process_task` never returns it throw an error, so the return is
always either `true` or an Error.
Document query options

I have documented only the option we currently use (:filter) and the option to the query engine that is disabled (:format). I have then referred to the documentation for the Fog module that defines these options (which is pretty comprehensive) for further information.
Merge pull request #137 from gds-operations/75691732-document-vcloud-core

[#75691732] Document vcloud-core public methods