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.