[vdsm] API Documentation & Since tag

Hi everyone,

We are currently documenting the API in vdsmapi-schema.json I noticed that we have there documented when a certain element newly is introduced using the 'Since' tag. However I also noticed that we are not documenting when a field was newly added, nor do we update the 'since' tag.

We should start documenting in what version we've introduced a field. A suggestion by saggi was to add to the comment for example: @since: 4.10.3

What is your point of view on this?