I'm trying to do the same thing, don't think there's an answer to what the possible values are or how the string gets created in the first place. I read this in the documentation for html_instructions here: https://developers.google.com/maps/documentation/directions/get-directions#DirectionsStep

Contains formatted instructions for this step, presented as an HTML text string. This content is meant to be read as-is. Do not programmatically parse this display-only content.