Improve documentation of vendor options

Improve documentation of vendor options (vivco, vivso and v6 vendor class and opts), for instance explain when vivso for a particular vendor is added in the response with its suboptions (including when there is a vivco for this vendor in the query).

For the code reorganization of the code e.g. move code from appendRequestedOptions to appendRequestedVendorOptions, or add new unit tests are in the scope of this ticket.

It includes and extends too #1745 (closed)